ChemComm View Article Online
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
COMMUNICATION
Cite this: Chem. Commun., 2015, 51, 2429 Received 25th November 2014, Accepted 1st January 2015 DOI: 10.1039/c4cc09424g
View Journal | View Issue
Theoretical exploration of MgH2 and graphene nano-flakes in cyclohexane: proposing a new perspective toward functional hydrogen storage material† Runze Liu,‡a Yinghe Zhao‡b and Tianshu Chu*ac
www.rsc.org/chemcomm
We studied the reaction mechanism of di-n-butylmagnesium decomposing into MgH2 in cyclohexane, and found a new route easier than famous b-hydride elimination. Further, we explored the dynamic behavior of graphene nano-flakes and MgH2 in cyclohexane, and gained new insights for efficient hydrogen storage material preparation.
Magnesium hydride (MgH2), on account of its cheapness and high volumetric and gravimetric capacity of hydrogen (110 g H2 L 1 and 7.6 wt% H2), has been considered as one of the most promising hydrogen storage materials.1–4 However, the main obstacles for the application of MgH2 are high dehydrogenation temperature and slow kinetics.4 Both theoretical and experimental studies have reported that nanoscale MgH2 clusters have much better hydrogen storage performance than bulk MgH2.5–11 The smaller size of colloidal MgH2 particles could destabilize the Mg–H bond, leading to a lower desorption temperature.8 The dominant approach in making MgH2 nanoparticles is ball-milling.3 Although ball-milled nanoscale materials can drastically enhance the de/hydrogenation rate which is mainly due to the surface enlargement, it is of little benefit to thermodynamics because the obtained MgH2 grain size is still large.8 Several limitations exist for the ball-milling method: a long milling time,1 difficulty in large-scale preparation1 and inability a
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academic of Sciences, Dalian 116023, P. R. China. E-mail: [email protected], [email protected] b Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China c Institute for Computational Sciences and Engineering, Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China † Electronic supplementary information (ESI) available: Computation details, the configurations of products and intermediates in cyclohexane, the analysis of intermolecular interactions, the comparison of two simulations with the same initial configuration and different force fields, the structure of graphene nanoflakes, and the trajectories of various simulations with different conditions. See DOI: 10.1039/c4cc09424g ‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2015
to control the morphology of the MgH2 nanoparticles,12 which impede the potential for further improving the hydrogen storage ability. Recently, a new method for the preparation of MgH2 nanoparticles has been reported through thermal decomposition of di-n-butylmagnesium, which could control the particle morphology in the nanometer range by altering synthesis parameters.12 Particularly, the MgH2 nanoparticles generated from hydrogenolysis of di-n-butylmagnesium in cyclohexane under hydrogen pressure exhibit a longer cycle life and a three-fold dehydrogenation rate compared with the ball-milled MgH2.12 Besides, this achievement of fast kinetics is not supported by any catalyst, leaving margins for designing more effective hydrogen storage materials with magnesium. However, one defect of this method is that the obtained cluster size is larger than 15 nm, resulting in the desorption temperature still higher than 300 1C. Some modification approaches are therefore needed to improve the hydrogen storage properties of MgH2 nanoparticles. It is known that the key factors for enhancing hydrogen storage properties of MgH2 are smaller grain size, beneficial nanostructure morphology and appropriate catalysts. Extensive research indicates that the combination of carbon materials and MgH2 can achieve better hydrogen storage performances.13–16 Particularly, the graphene sheet is known to be able to adsorb nano-clusters in solution, and thus serves as an excellent catalyst for MgH2 de/hydrogenation.14 Enlightened by this, we proposed a new idea of generating the graphene–MgH2 composite by utilizing graphene nano-flakes to adsorb MgH2 clusters by the decomposition of di-n-butylmagnesium in cyclohexane. Herein, we first articulated the mechanism of di-n-butylmagnesium decomposing into MgH2 in cyclohexane under hydrogen pressure by quantum mechanics (QM) calculations. Then we explored the dynamic behavior of graphene nano-flakes and MgH2 in cyclohexane by performing molecular dynamics (MD) simulations, which sheds light on the prospect of synthesizing new functional hydrogen storage materials based on MgH2. All computational details are described in ESI.† It is commonly believed that decomposition of di-n-butylmagnesium in cyclohexane is dominated by the well-known
Chem. Commun., 2015, 51, 2429--2432 | 2429
View Article Online
Communication
ChemComm
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
b-hydride elimination of metal alkyls, which involves the formation of a metal–H bond and a p bond of hydrocarbon.12,17 The reaction scheme is: C4H9–Mg–C4H9 - C4H9–Mg–H + C4H8
(1)
C4H9–Mg–H - MgH2 + C4H8
(2)
In terms of di-n-butylmagnesium, experiment shows that in the solid state it can directly decompose into MgH2 under an argon atmosphere,12 indicating the presence of b-hydride elimination. However, we cannot be certain if b-hydride elimination is dominant in cyclohexane under hydrogen pressure. In cyclohexane solution, the crystal phase di-n-butylmagnesium diffuses into many single molecules that frequently encounter H2 molecules. Therefore, we proposed a new two-step mechanism that involves a direct reaction with H2 molecules: C4H9–Mg–C4H9 + H2 - C4H9–Mg–H + C4H10
(3)
C4H9–Mg–H + H2 - MgH2 + C4H10
(4)
The b-elimination path was examined and shown in Fig. 1(a). First, the Mg–Ca bond and the Cb–H bond on one side of the molecule align to the same side of the Ca–Cb bond, forming a ‘‘syn coplanar’’ state. Then the coplanar hydrogen on Cb approaches Mg, while the metal and alkyl group get separated gradually. After reaching the transition state, this composite directly turns into products with the formation of the Mg–H bond and butylene. Then the other side of the molecule proceeds through the same way, leading to the final MgH2 product. Intrinsic reaction coordination calculations have been carried out to verify that the transition state connects the reactants and products along the reaction pathway. The calculated energy barriers for the two steps are 34.45 and 34.51 kcal mol 1 respectively, indicating that the sequential formation of each Mg–H bond is in the same rate. The other reaction routine (channel (3) and (4)), as shown in Fig. 1(b), also involves two steps with almost equal barriers in the transition state. In the channel (3) the H2 molecule approaches the Mg–Ca bond, leading to the breaking of H2 bond and formation of C–H and Mg–H bonds.
Fig. 1 Reaction pathways for decomposition of di-n-butylmagnesium into MgH2: (a) reaction 1a and 1b (b) reaction 2a and 2b.
2430 | Chem. Commun., 2015, 51, 2429--2432
This process renders a 25.35 kcal mol 1 barrier height, which is easier to occur compared with b-elimination. A similar process happens in the channel (4), and the corresponding energy barrier is 26.0 kcal mol 1. Experiment reveals that MgH2 generated by the decomposition of di-n-butylmagnesium in cyclohexane assembles into nanoscale clusters, which show fast de/hydrogenation kinetics and a long cycle life.12 We thought of further enhancing its hydrogen storage properties by involving graphene nano-flakes in the reaction system, on account of the catalytic effect of graphene material.14 Based on our understanding, the products involve n-butane, 1-butylene and MgH2 while the intermediate butyl(hydride) magnesium (C4H9–Mg–H) also exists in cyclohexane, and hence we simulated their dynamic behaviors in cyclohexane respectively (see Fig. S1 in ESI†). The results clearly reveal that n-butane, 1-butylene and C4H9–Mg–H disperse in cyclohexane, while MgH2 assembles into clusters, which is in qualitative accordance with the experiment.12 In order to accurately depict the dynamic behavior of MgH2 and graphene nanoflakes in cyclohexane, it is critical to determine the intermolecular interactions between cyclohexane molecules, between MgH2 molecules, between MgH2 and cyclohexane, as well as between graphene nano-flakes and MgH2 or cyclohexane. So we calculated these interactions in some typical configurations using the QM method, and then made a comparison with those delineated by the molecular mechanism (MM) method using amber force field (ff) parameters18 (see Fig. 2 and Fig. S2 in ESI†). We observed that amber ff mimics the QM results well except graphene–MgH2 intermolecular interaction (red dotted line in Fig. 2), which is too weak to reproduce the QM results (red solid line in Fig. 2). As a result, we re-fitted the red solid line in Fig. 2, and the fitting result is shown as a red dashed line in the same figure. The corresponding parameters to mimic the graphene–MgH2 intermolecular interaction are e = 2.16 kJ mol 1 and s = 0.31 nm. Based on that, we run two comparative simulations of MgH2 and graphene nano-flakes (Fig. S3a in ESI†) in cyclohexane, i.e., one with the combination of amber ff and re-fitted parameters (denoted as SI); the other with entire amber ff parameters (denoted as SII). The trajectories (see Fig. S4 in ESI†) obviously show that graphene nano-flakes can absorb MgH2 in SI but not in SII.
Fig. 2 (a) and (b) are schematic configurations of graphene–cyclohexane and graphene–MgH2. In (a), the line connected by two opposite carbon atoms of cyclohexane is parallel to the graphene plane, and r1 represents the vertical distance between two centers. The arrangement of MgH2 in (b) is similar to that of cyclohexane in (a). The solid (QM method) and dotted lines (MM method) in (c) are the intermolecular energy changes when scanning r1 and r2. The red dashed line records the result of the re-fitted red solid line.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
ChemComm
The only one difference between SI and SII is graphene–MgH2 intermolecular interaction, so the absorption effect stems from the strong interaction between MgH2 and graphene nano-flakes. By monitoring the trajectory of SI in Fig. 3a and b, we found that MgH2 molecules rapidly assemble into small clusters (circled by oval frames) on graphene nano-flakes. Note that the timescale (t1) for small MgH2 clusters forming on graphene nano-flake was only 0.3 ns (Fig. 3a and b), while no relatively large clusters exist in solution. In other words, graphene nanoflakes adsorb MgH2 in early time, which was much shorter than that (t2) for MgH2 molecules just aggregating into relatively large clusters circled by a circular frame (see Fig. 3c and d). Also, we constructed various systems covering different concentrations of MgH2 and sizes of graphene nano-flakes, and their dynamic trajectories are placed in Fig. S5 of ESI.† Those configurations offer a clear picture that MgH2 instantly aggregates into many tiny clusters in solution and then these clusters along with MgH2 in the single molecule state are absorbed by graphene nano-flakes. From the moment on, these MgH2 molecules on graphene nanoflake start to assemble into small clusters; meanwhile those in solution are continuously absorbed. The series of configurations show that the small clusters on graphene nano-flake are from gradual accumulation and not from direct absorption from solution. However, regardless of which system, t2 is much longer than t1, showing that the time order (t2 4 t1) is a common law. The order is a key point because it makes the combination of small clusters and graphene nano-flake possible. If extending to real system, it also should mean that MgH2 have been absorbed by graphene nano-flake when they have not yet shown obvious self-aggregation. Besides, according to Fig. 3 and Fig. S5 in ESI,† the number of small MgH2 clusters onto graphene nano-flake
Fig. 3 (a) is the initial configuration of MgH2 and graphene nano-flakes in cyclohexane (system V in computation details of ESI†), and (b) is dynamic evolution of (a) 0.3 ns later. (c) is (a) without the graphene nano-flakes, and (d) records dynamic evolution of (c) in 2.0 ns. They are all captured using VMD.19 The plate, ball, and point represent graphene nano-flakes, MgH2 and cyclohexane respectively.
This journal is © The Royal Society of Chemistry 2015
Communication
at same time (0.3 ns) increases with the higher MgH2 concentration and expansion of the flake. On the contrary, the size of cluster onto the flake shows little relation to the MgH2 concentration or flake area. It is noteworthy that the graphene nano-flake in this work is perfect, so each carbon atom of graphene nano-flakes is treated equally. Therefore, MgH2 clusters appear on the flake in a random manner (see Fig. S5 in ESI†), which is different from the cluster growth on graphene that has vacancy defects.20 Fast dynamic absorption causes MgH2 molecules in cyclohexane to rapidly approach graphene nano-flakes and then assemble into clusters onto them instead of self-assembling into big-size clusters in solution. In other words, the participation of graphene nano-flakes blocks the self-aggregation of MgH2, and accordingly they are distributed onto different graphene nano-flakes, further reducing the cluster size. Above all, it is theoretically practical to make the composite consisting of graphene nano-flakes and considerably small size MgH2 clusters (GFSMC) by combining them into cyclohexane solution under reasonable control over their ration or the area of graphene nano-flakes. It is known that a key factor of hydrogen storage properties is that the smaller size of the MgH2 cluster would result in a lower desorption temperature.5–11 On the other hand, Setijadi and coworkers have reported that cyclohexane solution benefits the morphology of the MgH2 cluster.12 The morphology of GFSMC should be similar to that of the MgH2 clusters generated without graphene nano-flakes in cyclohexane. Along with the catalytic effect of graphene nanoflakes on MgH2 desorption,14 we believe that GFSMC may be of lower desorption temperature (even o300 1C) compared to the current hydrogen storage materials based on MgH2. However, regrettably, the present MD theory level cannot give the final structure of GFSMC because it is impossible to simulate such a complex system in the experimental time and space scale. As a result, the hydrogen storage property of GFSMC still needs to be measured in experiment, although we assume the GFSMC has advantageous properties in view of small size, potentially beneficial morphology and the catalytic effect of graphene nano-flakes. In summary, we proposed a new idea of synthesizing functional hydrogen storage material based on graphene nanoflakes and small MgH2 clusters. First we studied the reaction mechanism of the di-n-butylmagnesium decomposing into MgH2 in cyclohexane and found that directly breaking the Mg–C bond with the participation of the H2 molecule has lower reaction energy barrier compared with b-hydride elimination. Then we carried out MD simulations, and verified that the graphene nano-flakes can absorb MgH2 molecules in cyclohexane. Based on that, we proposed a new hydrogen storage material on the theoretical side, which is composed of graphene nano-flakes and small size MgH2 clusters (GFSMC). It is potential to have better hydrogen storage properties21 compared to similar materials. This work was supported by the National Basic Research Program of China (2013CB834604) and NSFC (no. 21273234) and Shandong Provincial Natural Science Foundation, China (no. ZR2014AM025).
Chem. Commun., 2015, 51, 2429--2432 | 2431
View Article Online
Communication
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
Notes and references 1 B. Sakintuna, F. Lamari-Darkrim and M. Hirscher, Int. J. Hydrogen Energy, 2007, 32, 1121. 2 L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294. 3 I. P. Jain, C. Lal and A. Jain, Int. J. Hydrogen Energy, 2010, 35, 5133. 4 K. F. Aguey-Zinsou and J. R. Ares-Fernandez, Energy Environ. Sci., 2010, 3, 526. 5 K. F. Aguey-Zinsou and J. R. Ares-Fernandez, Chem. Mater., 2008, 20, 376. 6 M. Paskevicius, D. A. Sheppard and C. E. Buckley, J. Am. Chem. Soc., 2010, 132, 5077. 7 S. Harder, J. Spielmann, J. Intemann and H. Bandmann, Angew. Chem., Int. Ed., 2011, 50, 4156. 8 R. W. P. Wagemans, J. H. van Lenthe, P. E. de Jongh, A. J. van Dillen and K. P. de Jong, J. Am. Chem. Soc., 2005, 127, 16675–16680. 9 Z. G. Wu, M. D. Allendorf and J. C. Grossman, J. Am. Chem. Soc., 2009, 131, 13918. 10 Z. Zhao-Karger, J. J. Hu, A. Roth, D. Wang, C. Kubel, W. Lohstroh and M. Fichtner, Chem. Commun., 2010, 46, 8353.
2432 | Chem. Commun., 2015, 51, 2429--2432
ChemComm 11 V. Berube, G. Chen and M. S. Dresselhaus, Int. J. Hydrogen Energy, 2008, 33, 4122. 12 E. J. Setijadi, C. Boyer and K. F. Aguey-Zinsou, Int. J. Hydrogen Energy, 2013, 38, 5746. 13 A. J. Du, S. C. Smith, X. D. Yao and G. Q. Lu, J. Phys. Chem. B, 2006, 110, 1814. 14 G. Liu, Y. J. Wang, C. C. Xu, F. Y. Qiu, C. H. An, L. Li, L. F. Jiao and H. T. Yuan, Nanoscale, 2013, 5, 1074. 15 Z. G. Huang, Z. P. Guo, A. Calka, D. Wexler and H. K. Liu, J. Alloys Compd., 2007, 427, 94. 16 C. Z. Wu, P. Wang, X. Yao, C. Liu, D. M. Chen, G. Q. Lu and H. M. Cheng, J. Alloys Compd., 2006, 414, 259. 17 M. Konarova, A. Tanksale, J. N. Beltramini and G. Q. Lu, Int. J. Hydrogen Energy, 2012, 37, 8370. 18 A. T. Guy, T. J. Piggot and S. Khalid, Biophys. J., 2012, 103, 1028. 19 W. Humphrey, A. Dalke and K. Schulten, J. Mol. Graphics, 1996, 14, 33. 20 W. Gao, J. E. Mueller, J. Anton, Q. Jiang and T. Jacob, Angew. Chem., Int. Ed., 2013, 52, 14237. 21 A. Li, R. F. Lu, Y. Wang, X. Wang, K. L. Han and W. Q. Deng, Angew. Chem., Int. Ed., 2010, 49, 3330.
This journal is © The Royal Society of Chemistry 2015
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
COMMUNICATION
Cite this: Chem. Commun., 2015, 51, 2429 Received 25th November 2014, Accepted 1st January 2015 DOI: 10.1039/c4cc09424g
View Journal | View Issue
Theoretical exploration of MgH2 and graphene nano-flakes in cyclohexane: proposing a new perspective toward functional hydrogen storage material† Runze Liu,‡a Yinghe Zhao‡b and Tianshu Chu*ac
www.rsc.org/chemcomm
We studied the reaction mechanism of di-n-butylmagnesium decomposing into MgH2 in cyclohexane, and found a new route easier than famous b-hydride elimination. Further, we explored the dynamic behavior of graphene nano-flakes and MgH2 in cyclohexane, and gained new insights for efficient hydrogen storage material preparation.
Magnesium hydride (MgH2), on account of its cheapness and high volumetric and gravimetric capacity of hydrogen (110 g H2 L 1 and 7.6 wt% H2), has been considered as one of the most promising hydrogen storage materials.1–4 However, the main obstacles for the application of MgH2 are high dehydrogenation temperature and slow kinetics.4 Both theoretical and experimental studies have reported that nanoscale MgH2 clusters have much better hydrogen storage performance than bulk MgH2.5–11 The smaller size of colloidal MgH2 particles could destabilize the Mg–H bond, leading to a lower desorption temperature.8 The dominant approach in making MgH2 nanoparticles is ball-milling.3 Although ball-milled nanoscale materials can drastically enhance the de/hydrogenation rate which is mainly due to the surface enlargement, it is of little benefit to thermodynamics because the obtained MgH2 grain size is still large.8 Several limitations exist for the ball-milling method: a long milling time,1 difficulty in large-scale preparation1 and inability a
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academic of Sciences, Dalian 116023, P. R. China. E-mail: [email protected], [email protected] b Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China c Institute for Computational Sciences and Engineering, Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China † Electronic supplementary information (ESI) available: Computation details, the configurations of products and intermediates in cyclohexane, the analysis of intermolecular interactions, the comparison of two simulations with the same initial configuration and different force fields, the structure of graphene nanoflakes, and the trajectories of various simulations with different conditions. See DOI: 10.1039/c4cc09424g ‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2015
to control the morphology of the MgH2 nanoparticles,12 which impede the potential for further improving the hydrogen storage ability. Recently, a new method for the preparation of MgH2 nanoparticles has been reported through thermal decomposition of di-n-butylmagnesium, which could control the particle morphology in the nanometer range by altering synthesis parameters.12 Particularly, the MgH2 nanoparticles generated from hydrogenolysis of di-n-butylmagnesium in cyclohexane under hydrogen pressure exhibit a longer cycle life and a three-fold dehydrogenation rate compared with the ball-milled MgH2.12 Besides, this achievement of fast kinetics is not supported by any catalyst, leaving margins for designing more effective hydrogen storage materials with magnesium. However, one defect of this method is that the obtained cluster size is larger than 15 nm, resulting in the desorption temperature still higher than 300 1C. Some modification approaches are therefore needed to improve the hydrogen storage properties of MgH2 nanoparticles. It is known that the key factors for enhancing hydrogen storage properties of MgH2 are smaller grain size, beneficial nanostructure morphology and appropriate catalysts. Extensive research indicates that the combination of carbon materials and MgH2 can achieve better hydrogen storage performances.13–16 Particularly, the graphene sheet is known to be able to adsorb nano-clusters in solution, and thus serves as an excellent catalyst for MgH2 de/hydrogenation.14 Enlightened by this, we proposed a new idea of generating the graphene–MgH2 composite by utilizing graphene nano-flakes to adsorb MgH2 clusters by the decomposition of di-n-butylmagnesium in cyclohexane. Herein, we first articulated the mechanism of di-n-butylmagnesium decomposing into MgH2 in cyclohexane under hydrogen pressure by quantum mechanics (QM) calculations. Then we explored the dynamic behavior of graphene nano-flakes and MgH2 in cyclohexane by performing molecular dynamics (MD) simulations, which sheds light on the prospect of synthesizing new functional hydrogen storage materials based on MgH2. All computational details are described in ESI.† It is commonly believed that decomposition of di-n-butylmagnesium in cyclohexane is dominated by the well-known
Chem. Commun., 2015, 51, 2429--2432 | 2429
View Article Online
Communication
ChemComm
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
b-hydride elimination of metal alkyls, which involves the formation of a metal–H bond and a p bond of hydrocarbon.12,17 The reaction scheme is: C4H9–Mg–C4H9 - C4H9–Mg–H + C4H8
(1)
C4H9–Mg–H - MgH2 + C4H8
(2)
In terms of di-n-butylmagnesium, experiment shows that in the solid state it can directly decompose into MgH2 under an argon atmosphere,12 indicating the presence of b-hydride elimination. However, we cannot be certain if b-hydride elimination is dominant in cyclohexane under hydrogen pressure. In cyclohexane solution, the crystal phase di-n-butylmagnesium diffuses into many single molecules that frequently encounter H2 molecules. Therefore, we proposed a new two-step mechanism that involves a direct reaction with H2 molecules: C4H9–Mg–C4H9 + H2 - C4H9–Mg–H + C4H10
(3)
C4H9–Mg–H + H2 - MgH2 + C4H10
(4)
The b-elimination path was examined and shown in Fig. 1(a). First, the Mg–Ca bond and the Cb–H bond on one side of the molecule align to the same side of the Ca–Cb bond, forming a ‘‘syn coplanar’’ state. Then the coplanar hydrogen on Cb approaches Mg, while the metal and alkyl group get separated gradually. After reaching the transition state, this composite directly turns into products with the formation of the Mg–H bond and butylene. Then the other side of the molecule proceeds through the same way, leading to the final MgH2 product. Intrinsic reaction coordination calculations have been carried out to verify that the transition state connects the reactants and products along the reaction pathway. The calculated energy barriers for the two steps are 34.45 and 34.51 kcal mol 1 respectively, indicating that the sequential formation of each Mg–H bond is in the same rate. The other reaction routine (channel (3) and (4)), as shown in Fig. 1(b), also involves two steps with almost equal barriers in the transition state. In the channel (3) the H2 molecule approaches the Mg–Ca bond, leading to the breaking of H2 bond and formation of C–H and Mg–H bonds.
Fig. 1 Reaction pathways for decomposition of di-n-butylmagnesium into MgH2: (a) reaction 1a and 1b (b) reaction 2a and 2b.
2430 | Chem. Commun., 2015, 51, 2429--2432
This process renders a 25.35 kcal mol 1 barrier height, which is easier to occur compared with b-elimination. A similar process happens in the channel (4), and the corresponding energy barrier is 26.0 kcal mol 1. Experiment reveals that MgH2 generated by the decomposition of di-n-butylmagnesium in cyclohexane assembles into nanoscale clusters, which show fast de/hydrogenation kinetics and a long cycle life.12 We thought of further enhancing its hydrogen storage properties by involving graphene nano-flakes in the reaction system, on account of the catalytic effect of graphene material.14 Based on our understanding, the products involve n-butane, 1-butylene and MgH2 while the intermediate butyl(hydride) magnesium (C4H9–Mg–H) also exists in cyclohexane, and hence we simulated their dynamic behaviors in cyclohexane respectively (see Fig. S1 in ESI†). The results clearly reveal that n-butane, 1-butylene and C4H9–Mg–H disperse in cyclohexane, while MgH2 assembles into clusters, which is in qualitative accordance with the experiment.12 In order to accurately depict the dynamic behavior of MgH2 and graphene nanoflakes in cyclohexane, it is critical to determine the intermolecular interactions between cyclohexane molecules, between MgH2 molecules, between MgH2 and cyclohexane, as well as between graphene nano-flakes and MgH2 or cyclohexane. So we calculated these interactions in some typical configurations using the QM method, and then made a comparison with those delineated by the molecular mechanism (MM) method using amber force field (ff) parameters18 (see Fig. 2 and Fig. S2 in ESI†). We observed that amber ff mimics the QM results well except graphene–MgH2 intermolecular interaction (red dotted line in Fig. 2), which is too weak to reproduce the QM results (red solid line in Fig. 2). As a result, we re-fitted the red solid line in Fig. 2, and the fitting result is shown as a red dashed line in the same figure. The corresponding parameters to mimic the graphene–MgH2 intermolecular interaction are e = 2.16 kJ mol 1 and s = 0.31 nm. Based on that, we run two comparative simulations of MgH2 and graphene nano-flakes (Fig. S3a in ESI†) in cyclohexane, i.e., one with the combination of amber ff and re-fitted parameters (denoted as SI); the other with entire amber ff parameters (denoted as SII). The trajectories (see Fig. S4 in ESI†) obviously show that graphene nano-flakes can absorb MgH2 in SI but not in SII.
Fig. 2 (a) and (b) are schematic configurations of graphene–cyclohexane and graphene–MgH2. In (a), the line connected by two opposite carbon atoms of cyclohexane is parallel to the graphene plane, and r1 represents the vertical distance between two centers. The arrangement of MgH2 in (b) is similar to that of cyclohexane in (a). The solid (QM method) and dotted lines (MM method) in (c) are the intermolecular energy changes when scanning r1 and r2. The red dashed line records the result of the re-fitted red solid line.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
ChemComm
The only one difference between SI and SII is graphene–MgH2 intermolecular interaction, so the absorption effect stems from the strong interaction between MgH2 and graphene nano-flakes. By monitoring the trajectory of SI in Fig. 3a and b, we found that MgH2 molecules rapidly assemble into small clusters (circled by oval frames) on graphene nano-flakes. Note that the timescale (t1) for small MgH2 clusters forming on graphene nano-flake was only 0.3 ns (Fig. 3a and b), while no relatively large clusters exist in solution. In other words, graphene nanoflakes adsorb MgH2 in early time, which was much shorter than that (t2) for MgH2 molecules just aggregating into relatively large clusters circled by a circular frame (see Fig. 3c and d). Also, we constructed various systems covering different concentrations of MgH2 and sizes of graphene nano-flakes, and their dynamic trajectories are placed in Fig. S5 of ESI.† Those configurations offer a clear picture that MgH2 instantly aggregates into many tiny clusters in solution and then these clusters along with MgH2 in the single molecule state are absorbed by graphene nano-flakes. From the moment on, these MgH2 molecules on graphene nanoflake start to assemble into small clusters; meanwhile those in solution are continuously absorbed. The series of configurations show that the small clusters on graphene nano-flake are from gradual accumulation and not from direct absorption from solution. However, regardless of which system, t2 is much longer than t1, showing that the time order (t2 4 t1) is a common law. The order is a key point because it makes the combination of small clusters and graphene nano-flake possible. If extending to real system, it also should mean that MgH2 have been absorbed by graphene nano-flake when they have not yet shown obvious self-aggregation. Besides, according to Fig. 3 and Fig. S5 in ESI,† the number of small MgH2 clusters onto graphene nano-flake
Fig. 3 (a) is the initial configuration of MgH2 and graphene nano-flakes in cyclohexane (system V in computation details of ESI†), and (b) is dynamic evolution of (a) 0.3 ns later. (c) is (a) without the graphene nano-flakes, and (d) records dynamic evolution of (c) in 2.0 ns. They are all captured using VMD.19 The plate, ball, and point represent graphene nano-flakes, MgH2 and cyclohexane respectively.
This journal is © The Royal Society of Chemistry 2015
Communication
at same time (0.3 ns) increases with the higher MgH2 concentration and expansion of the flake. On the contrary, the size of cluster onto the flake shows little relation to the MgH2 concentration or flake area. It is noteworthy that the graphene nano-flake in this work is perfect, so each carbon atom of graphene nano-flakes is treated equally. Therefore, MgH2 clusters appear on the flake in a random manner (see Fig. S5 in ESI†), which is different from the cluster growth on graphene that has vacancy defects.20 Fast dynamic absorption causes MgH2 molecules in cyclohexane to rapidly approach graphene nano-flakes and then assemble into clusters onto them instead of self-assembling into big-size clusters in solution. In other words, the participation of graphene nano-flakes blocks the self-aggregation of MgH2, and accordingly they are distributed onto different graphene nano-flakes, further reducing the cluster size. Above all, it is theoretically practical to make the composite consisting of graphene nano-flakes and considerably small size MgH2 clusters (GFSMC) by combining them into cyclohexane solution under reasonable control over their ration or the area of graphene nano-flakes. It is known that a key factor of hydrogen storage properties is that the smaller size of the MgH2 cluster would result in a lower desorption temperature.5–11 On the other hand, Setijadi and coworkers have reported that cyclohexane solution benefits the morphology of the MgH2 cluster.12 The morphology of GFSMC should be similar to that of the MgH2 clusters generated without graphene nano-flakes in cyclohexane. Along with the catalytic effect of graphene nanoflakes on MgH2 desorption,14 we believe that GFSMC may be of lower desorption temperature (even o300 1C) compared to the current hydrogen storage materials based on MgH2. However, regrettably, the present MD theory level cannot give the final structure of GFSMC because it is impossible to simulate such a complex system in the experimental time and space scale. As a result, the hydrogen storage property of GFSMC still needs to be measured in experiment, although we assume the GFSMC has advantageous properties in view of small size, potentially beneficial morphology and the catalytic effect of graphene nano-flakes. In summary, we proposed a new idea of synthesizing functional hydrogen storage material based on graphene nanoflakes and small MgH2 clusters. First we studied the reaction mechanism of the di-n-butylmagnesium decomposing into MgH2 in cyclohexane and found that directly breaking the Mg–C bond with the participation of the H2 molecule has lower reaction energy barrier compared with b-hydride elimination. Then we carried out MD simulations, and verified that the graphene nano-flakes can absorb MgH2 molecules in cyclohexane. Based on that, we proposed a new hydrogen storage material on the theoretical side, which is composed of graphene nano-flakes and small size MgH2 clusters (GFSMC). It is potential to have better hydrogen storage properties21 compared to similar materials. This work was supported by the National Basic Research Program of China (2013CB834604) and NSFC (no. 21273234) and Shandong Provincial Natural Science Foundation, China (no. ZR2014AM025).
Chem. Commun., 2015, 51, 2429--2432 | 2431
View Article Online
Communication
Published on 01 January 2015. Downloaded by University of California - San Diego on 02/02/2015 12:16:56.
Notes and references 1 B. Sakintuna, F. Lamari-Darkrim and M. Hirscher, Int. J. Hydrogen Energy, 2007, 32, 1121. 2 L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294. 3 I. P. Jain, C. Lal and A. Jain, Int. J. Hydrogen Energy, 2010, 35, 5133. 4 K. F. Aguey-Zinsou and J. R. Ares-Fernandez, Energy Environ. Sci., 2010, 3, 526. 5 K. F. Aguey-Zinsou and J. R. Ares-Fernandez, Chem. Mater., 2008, 20, 376. 6 M. Paskevicius, D. A. Sheppard and C. E. Buckley, J. Am. Chem. Soc., 2010, 132, 5077. 7 S. Harder, J. Spielmann, J. Intemann and H. Bandmann, Angew. Chem., Int. Ed., 2011, 50, 4156. 8 R. W. P. Wagemans, J. H. van Lenthe, P. E. de Jongh, A. J. van Dillen and K. P. de Jong, J. Am. Chem. Soc., 2005, 127, 16675–16680. 9 Z. G. Wu, M. D. Allendorf and J. C. Grossman, J. Am. Chem. Soc., 2009, 131, 13918. 10 Z. Zhao-Karger, J. J. Hu, A. Roth, D. Wang, C. Kubel, W. Lohstroh and M. Fichtner, Chem. Commun., 2010, 46, 8353.
2432 | Chem. Commun., 2015, 51, 2429--2432
ChemComm 11 V. Berube, G. Chen and M. S. Dresselhaus, Int. J. Hydrogen Energy, 2008, 33, 4122. 12 E. J. Setijadi, C. Boyer and K. F. Aguey-Zinsou, Int. J. Hydrogen Energy, 2013, 38, 5746. 13 A. J. Du, S. C. Smith, X. D. Yao and G. Q. Lu, J. Phys. Chem. B, 2006, 110, 1814. 14 G. Liu, Y. J. Wang, C. C. Xu, F. Y. Qiu, C. H. An, L. Li, L. F. Jiao and H. T. Yuan, Nanoscale, 2013, 5, 1074. 15 Z. G. Huang, Z. P. Guo, A. Calka, D. Wexler and H. K. Liu, J. Alloys Compd., 2007, 427, 94. 16 C. Z. Wu, P. Wang, X. Yao, C. Liu, D. M. Chen, G. Q. Lu and H. M. Cheng, J. Alloys Compd., 2006, 414, 259. 17 M. Konarova, A. Tanksale, J. N. Beltramini and G. Q. Lu, Int. J. Hydrogen Energy, 2012, 37, 8370. 18 A. T. Guy, T. J. Piggot and S. Khalid, Biophys. J., 2012, 103, 1028. 19 W. Humphrey, A. Dalke and K. Schulten, J. Mol. Graphics, 1996, 14, 33. 20 W. Gao, J. E. Mueller, J. Anton, Q. Jiang and T. Jacob, Angew. Chem., Int. Ed., 2013, 52, 14237. 21 A. Li, R. F. Lu, Y. Wang, X. Wang, K. L. Han and W. Q. Deng, Angew. Chem., Int. Ed., 2010, 49, 3330.
This journal is © The Royal Society of Chemistry 2015
