The Formation of CH4 Hydrate in the Slit Nanopore between the

Subscriber access provided by UNIV OF NEWCASTLE

Fossil Fuels

The Formation of CH4 Hydrate in the Slit Nanopore between the Smectite Basal Surfaces by Molecular Dynamics Simulation Ke-Feng Yan, Xiao-Sen Li, Zhao-Yang Chen, Chun-Gang Xu, Yu Zhang, and Zhi-Ming Xia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00195 • Publication Date (Web): 13 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The Formation of CH4 Hydrate in the Slit Nanopore between the Smectite Basal Surfaces by Molecular Dynamics Simulation Kefeng Yan,†,‡,§,ǁ Xiaosen Li,*, †,‡,§,ǁ Zhaoyang Chen,†,‡,§,ǁ Chungang Xu,†,‡,§,ǁ Yu Zhang,†,‡,§,ǁ and Zhiming Xia†,‡,§,ǁ †

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy

of Sciences, Guangzhou 510640, PR China ‡

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

Development, Guangzhou 510640, PR China §

University of Chinese Academy of Sciences, Beijing 100049, PR China

ǁ

Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou

510640, PR China

ABSTRACT: Natural gas hydrate (NGH) formation behavior in porous sediment influences on the investigation of the reservoirs and the exploitation of NGH. However, its molecular mechanisms of NGH formation in the porous sediment remain unclear. In the work, we present the CH4 hydrate formation in the smectite system through molecular dynamics simulation. The microstructure molecular configurations and properties are analyzed. The results find the pure

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

H2O solution and the CH4-H2O homogeneous solution in the initial configuration of the smectite layer reveal the different influence on the hydrate formation. The gas-water ratio (ri) affects the molecular diffusion and the hydrate formation. In the smectite layer, the two type arrangements of cages are present: the semi-cage arrangement and the link-cage arrangement. The siliconoxygen ring of the smectite surface connecting with the two type arrangements of cages has the stable effect for the hydrate formation in the smectite.

KEYWORDS: smectite; CH4 hydrate; molecular dynamics simulation; formation behavior

1. INTRODUCTION Natural gas hydrates (NGHs) widely distribute in the permafrost areas and ocean sediment. It is a new energy source on the earth. The formation behavior of NGH in porous sediment influences on the researches of the exploitation and the reservoirs of NGH. Recently, the phase equilibrium and kinetic of the hydrate formation in the porous sediment are investigated by the experiments and the mathematical simulations.1-6 Smectite was also been investigated by the hydrate formation experiments representing as a kind of the porous sediment.7-9 Experiments proved the condition of the hydrate formation was milder in the smectite than that in the pure water.10, 11 In the smectite, the mechanism of hydrate formation, however, was far less clearly understood by the mathematical simulation studies and the experimental studies. As a computational tool at the molecular scale, molecular dynamics simulation (MD) has been applied to understand of the crystal formation mechanisms of the gas hydrate

12-15

. In the pure

water system, the mechanisms of gas hydrate formation were carried out by MD.12-15 The results indicated there were different hypotheses to explain the hydrate formation behavior in the bulk solution. In the porous sediment system, the processes of the hydrate formation in the present of


Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

silica system were carried out by MD.16-18 The results indicated H2O molecules structures in the vicinity of the silica surface were ice-like. In the silica surfaces, the hydroxyl can facilitate the gas hydrate growth. The smectite could be considered as a possible existence of the NGH widely distributing in marine sediment. The formation mechanism of gas hydrate in the smectite is quite important to the studies of the NGH formation behavior in marine sediment. Recently, the MD simulations and Monte Carlo (MC) were employed to investigate the stable interlayer CH4 hydrate structures in the smectite.8, 19-21 The simulation results found the structure of CH4 clathrate hydrate in the smectite was different from that in the pure water. CH4 molecules were nested on the hexagonal ring of the smectite surface oxygen, and constructed a hydrate-like structure with H2O molecules. In the smectite, the density of H2O solution was lower than that in the pure water. H2O molecules insufficiently offered the coordination to CH4 molecules. The sufficient interlayer water therefore was an important factor for the hydrate formation. Therefore, in the work, we perform MD to investigate the CH4 hydrate formation processes in the smectite with the pure water and the CH4-H2O homogeneous solution. The microstructure molecular configuration and properties are reported. We discuss the effect of gas-water ratio (ri) for hydrate formation.

2. METHODOLOGY As the model smectite, we use the generic Wyoming type Na-montmorillonite, which is constructed from our prior simulations.22, 23 The molecular formula of the Na-montmorillonite is Na3(Si31Al)(Al14Mg2)O80(OH)16. In the work, the 6 H2O layer hydrate intercalates in the smectite, which is defined from the previous researches.24-26 The simulated system is a bulk CH4-H2O


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

solution layer in contact with a smectite layer alone the (0 1 0) face of the smectite (Figure 1). The smectite layer along the z-direction includes the supercell (1×1×2) of Na-montmorillonite model. The simulation cell is corresponding to the basal spacing of 6 H2O layer hydrate along xdirection. The edges of the smectite is created by cutting the (0 1 0) face of the smectite layer. And in the smectite edge, the broken bonds are saturated by the hydroxyl groups.27,

28

The

dimensions of the simulation system are 2.50×2.11×7.40 nm3 in the x, y, z directions. The region of the bulk solution layer is about 0~3.60 nm and the region of the smectite layer is about 3.60~7.40 nm at z direction. In this work, there are three simulations to run. Figure 1 shows the initial configurations of three simulation systems. At Run 1, the initial configuration is a bulk solution layer in contact with a smectite layer intercalated the pure H2O solution. The CH4-H2O solution presents in the bulk solution layer, which density is about 0.948 g/cm3. This density is agree with the density of the structure I (sI) CH4 hydrate,29 and the ri in the bulk solution layer of Run 1 is same as the ri in the sI CH4 hydrate. The pure H2O solution presents in smectite layer of Run1, which density is about 0.97 g/cm3. This density conforms to the density of the H2O solution in smectite of the previous study.26 In our previous works confirmed that these densities are of benefit to the research of the hydrate formation in the smectite.23 At Run 2, the initial configuration is obtained by simulation a CH4-H2O homogeneous mixture to spontaneously form the CH4-H2O homogeneous solution in the bulk solution layer and the smectite layer at 300 K and 10 MPa for 200 ns. The total numbers of H2O molecules and CH4 molecules in the system of Run 2 are same as those in the system of Run 1. At Run3, some CH4 molecules are added to the initial configuration of Run 2 in the bulk solution layer representing as the initial configuration of Run3, which is for the investigation of the effect of ri for hydrate formation. The numbers of CH4


Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

molecules of Run 3 In the bulk solution layer is same as that of CH4 molecules of Run 1. Table 1 displays the CH4 molecules numbers and H2O molecules numbers in the simulation systems. DL_POLY package is used to perform all the MD simulations.30 The CLAYFF force field31 is employed for the smectite, which was widely used in smectite related studies.19, 20, 32 In the edge of the smectite layer, the force field parameters of the coordinated-hydroxyl are allocated by the hydrogen atom and hydroxyl oxygen types from the CLAYFF force field. The partial charges are modified slightly for balance the electrostatic charges of the smectite. This force field of smectite in the simulations is same as the previous studies33. H2O molecules and CH4 molecules are modeled using the TIP4P H2O model34 and the optimized potentials for liquid simulations AllAtom (OPLS-AA) Force Field35, respectively. These models have been successful used to investigate the hydrate formation in previous studies.36-39 The Lorentz–Berthelot combining rules is applied to calculate the Lennard-Jones potentials interaction parameters between different atom types. A cutoff distance for the short-ranged forces is 1 nm. And the Ewald summation method is applied to calculate the long-range electrostatic interactions.40 The Verlet Leapfrog algorithm is applied to integrate the equations of motion for rotations and translations with a timestep of 1 fs. Periodic boundary conditions are imposed along all three spatial directions. The simulations apply the Nose-Hoover algorithm to maintain temperature at 260 K. NPT relaxation process (2 ns) at 260 K and 10 MPa is run to remove the influence of the initial configuration. The next NVT fixed smectite molecules process is performed with the timescale of 1 µs. Thought, as smectite is able to swell or shrink as the environmental solution changes, there is little effect on the spacing of the smectite layer by the few H2O molecules diffusion between the bulk solution and the smectite layer. And the volume with the small fluctuation is relaxation in the initial NPT simulation. This fluctuation involves in the variation of cell along x-direction, y-


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

direction and z-direction. This variation is normal and has less influence for the result of the simulations. The hydrate structures of the simulation systems are characterized by computing the evolution of the four-body order parameter (F4φOP)

41, 42

and the Mutually Coordinated Guest

order parameter (MCGOP) 43 for H2O molecules, which have been successfully investigated into the hydrate formation and dissociation.14, 16, 44, 45

3. RESULTS AND DISCUSSION 3.1. The clathrate formation in the smectite layer The simulations of the CH4 hydrate formation have been carried out in the systems including a smectite layer and a bulk solution layer. Figure 2 shows the molecular configurations of the simulations at 1µs. In Run 1, it can be seen from Figure 1 and Figure 2 that CH4 molecules move from the bulk solution layer to the smectite layer. This molecular diffusion is due to the density gradient of CH4 molecules between the smectite layer and the bulk solution layer in the system of Run 1. In the bulk solution of the initial simulation, some CH4 molecules aggregate to form the bubble. Due to the two dimensional figure, some bubbles are difficult to visible Figure 1. CH4 molecules subsequently dissolve in the solution as the molecular diffusion, and then the CH4 bubbles disappear. In a smectite layer, the smectite molecules present the asymmetrical TOT structure.19 Some H2O molecules intercalate the smectite nanopore. With the diffused CH4 molecules these H2O molecules are form CH4-H2O solution in the smectite layer of Run 1. During the simulation of Run 1, the amorphous cluster of the CH4-H2O solution continually forms the irregular cages by arranging H2O molecules. The irregular cages are rearranged until the completely hydrate structure is formed. As the prior study46 reveals the potential energy of CH4 molecules in the neighboring of the solid surface is higher than the potential energy of CH4


Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

molecules in the liquid phase, CH4 molecules adsorb on the smectite surface. It can motivate the CH4 molecules diffusion and hydrate nucleation. However, while some clathrate form in the entrance of the smectite, the mass transfer barrier increases for CH4 molecules to diffuse from the bulk solution layer to the smectite layer. And it observably slows down the growth of the hydrate in the smectite layer. Therefore, from Figure 2 it is obvious the distribution of CH4 molecules in the bulk solution layer of Run 1is more than that in the smectite layer of Run 1. Then, the ri in the bulk solution layer of Run 1is higher than that in the smectite layer of Run 1, which is shown in Table 1. The previous works indicated that ri is a key factor to the hydrate formation in the bulk solution. And the condition of high ri can promote the hydrate formation.47 So, the hydrate can easier to form in the bulk solution layer of Run 1 than that in the smectite layer of Run 1. In Figure 2, we can see some amorphous cluster of H2O molecules and CH4 molecules arrange to form the cages in the bulk solution layer of Run 1, and few cages are formed in the smectite layer of Run 1. In Run 2, the CH4-H2O homogeneous solution presents in the bulk solution layer and the smectite layer. In the bulk solution layer, we can see from Table 1 that the ri of Run 2 is less than the ri of the ideal CH4 hydrate (0.174) at the initial simulation. During the simulation of Run 2, some empty cages form in the bulk solution layer due to the lowed ri in this region. Therefore, some CH4 molecules move from the smectite layer to the bulk solution layer for filling these empty cages in the system of Run 2. In the same time, the ri in the smectite layer of Run 2 is reduced for this molecular diffusion. After the simulation, it is obvious from Figure 2 that there is little cage form in the smectite layer of Run 2. The result indicates the reduced ri influence the hydrate formation both in the bulk solution layer and in the smectite layer. In Run 3, the ri in the initial bulk solution layer of Run 3 is relatively higher than that in the initial bulk solution layer


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

of Run 2, as shown in Table 1. And the numbers of empty cages relatively reduce. Then, it is less molecular diffusion between the bulk solution layer and the smectite layer. At 1 µs simulation of Run 3 in the bulk solution layer, the ri is 0.161, which is closed to the ri of the CH4 hydrate in sI crystal structure (0.174). At the same time, the ri in the smectite layer of Run 3 is more than the ri in the smectite layer of Run 1 and Run 2 at 1 µs simulation. Therefore, the system of Run 3 may easy to form the hydrate both in the smectite layer and in the bulk solution layer. To detail describe the status of and hydrate formation in the smectite layer of the simulation systems, the radial distribution functions (RDFs) of the simulation systems in the smectite layer are analyzed. In Figure 3, the gOwtr-Owtr(r) presents the RDF between O atoms of H2O molecules, and the gC-C(r) presents the RDF between C atoms of CH4 molecules in the smectite layer. From Figure 3(a), we can see the first, second and third peaks for the gOwtr-Owtr(r)s of three simulation systems in the smectite layer agree with that of the gOwtr-Owtr(r) for the pure CH4 hydrate.48, 49 it is indicated that the clathrate-like arrangement of H2O molecules in the smectite layer of three simulation systems. Comparing with the gOwtr-Owtr(r)s in the smectite layer of three simulation systems, we can found the second peak and third peak of gOwtr-Owtr(r)s of Run 1, Run 2 are broader than that of Run 3 in the smectite layer. It is indicated the arrangement of H2O molecules of Run 3 is more regular than that of Run 1 and Run 2. In Figure 3(b), the clear different profile for gC-C(r) of three simulation systems in the smectite layer can be found. While the first peak value in the smectite layer of Run 3 is closed to the first peak value of gC-C(r) in the pure CH4 hydrate,48, 49 the first peak value in the smectite layer of Run 1 and Run 2 is closed to the first peak value of gC-C(r) in liquid phase.

50

The second peak

values of gC-C(r)s of Run 1 and Run 2 is closed to the first peak value of gC-C(r) in the pure CH4 hydrate. The result indicates the arrangement of molecules of Run 3 in the smectite layer is more


Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

close to the arrangement of molecules in the pure CH4 hydrate than those of Run 1 and Run 2 in the smectite layer. In above analysis, in the systems of three simulation systems, many cages of Hydrate form in the bulk solution layer, while few cage of hydrate form in the smectite layer. Comparing with three simulation systems, it is found that the crystal hydrate of Run 3 is more regular than that of Run 1 and Run 2 in the smectite layer. We can draw the conclusion that the more clathrate-like structures in the smectite layer are formed with high ri. The ri is one key factor to influence hydrate formation in the porous sediment. 3.2. The arrangement of cage in the smectite layer Clearly displaying the crystal hydrate structure, some molecules of Run 3 at 1µs are deleted to exhibit the integrated cage configurations, which are shown in Figure 4. In the bulk solution layer, we can see from Figure 4 that some cages form. And these cages mainly present the 512 (12 pentagonal faces) and 51262 cages (12 pentagonal and 2 hexagonal faces), with some empty cages and some half-complete cages. In the edge of the smectite surface, the ice-like structural H2O molecules connects the hydrate in the bulk solution layer with the smectite surface. At the same time, the hydroxyls of the smectite surface edge sites and the ice-like structural H2O molecules cooperatively construct the stable semi-cage structures on the edge of the smectite surface with the adsorbed CH4 molecules. The semi-cages on the edge surface connect the 512 cage or the 51262 cage on the bulk solution layer through sharing a pentagonal face or a hexagonal face. In the smectite layer, some “interlayer cages” are formed by CH4 molecule and “interlayer H2O molecules”, while some “surface cages” are formed by “surface H2O molecules”, CH4 molecule and the silicon-oxygen rings of the smectite surface.22 From the red ring section of Figure 4(a), we find there are two type arrangements of the cages in the smectite layer. One is the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

semi-cage arrangement, and other is the link-cage arrangement, as shown in Figure 4(b). From the left in Figure 4(b), it can be seen some “surface H2O molecules” construct the network by Hbond with a silicon-oxygen ring of the smectite surface to form a semi–cage, which structure is like the structure of a 51262 cage in sI hydrate. At the same time, some “interlayer H2O molecules” construct the network by H-bond to form a 51262 cage. A semi-cage connects a 51262 cage through sharing a hexagonal face to construct the semi-cage arrangement, which arrangement is like the arrangement of sI hydrate (seen in the middle in Figure 4(b)). The linkcage arrangement is that the cages link with the silicon-oxygen rings of the smectite surface by Hydrogen bond. In the right of Figure 4(b), it can be shown some H2O molecules construct the network by H-bond to form a 512 cage. A 512 cage connects another cage through sharing a pentagonal face. The cages link with the silicon-oxygen rings of the smectite surface by Hydrogen bond to construct the link-cage arrangement. Though the arrangement of cages in the hydrate formation is stochastic and the cages in the smectite layer have defect caused by the steric effect of the cations, the semi-cage arrangement and the link-cage arrangement can present the two typical arrangements of the cages in the smectite layer. In the smectite layer, it is no distinct difference for the formation time between the “interlayer cages” and the “surface cages”. Therefore, there no obvious temporal sequence for the two type arrangements, though the linked surface of the smectite is different. Certainly, there may be a regularity of distribution between the two type arrangements in the smectite layer. However, it is necessary a long simulation time to form the complete hydrate in the smectite layer. As shown in Figure 2 and Figure 7, it is difficult to form the integral cage in the internal smectite nanopore region. Therefore, it is difficult to draw conclusions for the regularity of distribution of the two type arrangements in this work. The analyses of a ration between these two type arrangements in the smectite layer


Page 10 of 28

Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

would be done in the next work at a long time simulation, such as the timescale of several hundred microseconds. In this simulation, the two type arrangements make the hydrate structure stabilized in the smectite layer by connecting the silicon-oxygen rings of the smectite surface. It was inferred that the silicon-oxygen rings of the smectite surface have the stable effects for the hydrate formation in the smectite nanopore, which result is same the result of prior researches.8, 23 Figure 5 shows the RDFs of Run 3 in the smectite layer. From Figure 5, it can be seen the first peak is found at about 0.28 nm in the RDF between O atoms of silicon-oxygen ring (Osmectite) and O atoms of H2O molecules (Owtr) in the smectite layer, corresponding to the distance between the hydrogen bond of two H2O molecules. The result means the distance between Osmectite and Owtr of “surface H2O molecules” is same as the distance between Owtr of cages. A similar phenomenon is presented in the RDF between C atoms of CH4 molecules (C) and Osmectite in the smectite layer, as seen in Figure 5. The first peak in the RDF between C and Osmectite is corresponding to the distance between C and Owtr in the cages. Therefore, the silicon-oxygen rings of the smectite surface interact with H2O molecules and improve the hydrate formation. 3.3. The order parameter of H2O molecules on the hydrate formation The order parameter of H2O molecules can make identification of the hydrate formation events, which has the high resolution traces to the hydrate formation. F4φOP is a function of the torsion angle between O atoms of the adjacent H2O-H2O pairs within 0.35 nm and the outermost H atoms.41,

42

F4φOP can make identification of the liquid, ice and hydrate structures in

simulation system. The average F4φOP of liquid is -0.04, the average F4φOP of ice is 0.4 and the average F4φOP of hydrate structures is 0.7.51 The F4φOP is 0.4 as the location of the interface between liquid and solid hydrate. F4φOP for the trajectory of the simulations in the smectite layer


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and in the bulk solution layer are shown in Figure 6. We can see from Figure 6(a) the precipitous rises of the F4φOPs of three simulation systems reveal the hydrate formation process in the bulk solution layer. It is indicated the arrangement of H2O molecules in the bulk solution layer change from liquid to solid. As shown in Figure 6(a), the induction times for hydrate formation in the bulk solution layer of simulation systems are different. This phenomenon is due to the randomness of hydrate formation in the simulations. In the smectite layer, as shown in Figure 6(b), F4φOPs of three simulation systems are below 0.4. The results mean there is no entirely hydrate to form in three simulation systems in the smectite layer during 0~1 µs period. However, the uptrend of F4φOP of Run 3 indicates that the arrangements of H2O molecules of Run 3 trend to transform from liquid to solid in the smectite layer. It shows the hydrate formation process of Run 3 is different with the other systems in the smectite layer. Figure 7 shows the z-profile for the F4φOPs of the simulation systems. The F4φOP profile along the z axis is computed as an average over 10 ps during 990~1000 ns interval layer used for selecting particular water molecules in slices of 0.5 nm thicknesses. From Figure 7, we can see the values of the F4φOP of the simulation systems in the bulk solution layer are approximately closed to the value in the hydrate structure (0.7). The result implies H2O molecules of the simulation systems in this region present the solid structure. Same as the result of previous study,23 the F4φOPs on the interface of the smectite layer and the bulk solution layer fall suddenly. It is attributed to the influence of the edge sites of the smectitie surface for the torsion angle of the H2O-H2O pairs. Then, the F4φOPs rise to the basic line of the solid H2O (4.0) again in the smectite layer of the simulation systems, which implies H2O molecules arrange as the solid state in the entrance of the smectite layer. It would be formed some cages in this region. In the internal smectite nanopore region (z>4.5 nm), the F4φOPs fall under the basic line of the solid H2O (4.0).


Page 12 of 28

Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The result confirms that it is difficult to form the integral cage in the smectite layer caused by the low ri in this region. Comparing the z-profile for the F4φOPs of three simulation systems, we can find the F4φOP in the system of Run 3 is higher than those in the system of Run 1 and Run 2 in the internal smectite nanopore region. It indicates that the cages of Run 3 are more regular than those cages of Run 1 and Run 2 in the smectite layer. The MCGOP is a new order parameter for tracking hydrate formation, which have high identification traces to detect and quantify the hydrate formation. MCG monomer is considered as a criterion of hydrate formation. In the simulation systems, CH4 molecules are the guest molecules. The minimum number of H2O molecules Nw require to satisfy the following constraints: the distance of guest-guest Rgcut is 9 Å, the distance of guest-water Rwcut is 6 Å, and the specified angle φ of guest-guest vector is 45o. If Nw is 5, the number of candidate guest is added to the “count” (Nc). If the minimum number of Nc is 3, the candidate guest is considered as a MCG monomer. Figure 8 shows MCGOPs for the trajectory of the simulations in the bulk solution layer and in the smectite layer. From Figure 8(a), it can be seen some MCG monomers of three simulation systems display in the bulk solution layer. It is implied the crystal hydrate form in this region of three simulation systems. In the smectite layer, as seen in Figure 8(b), the MCGOPs of Run 1, Run 2 gradually appear during 0~1µs period, which indicates few MCG monomers of Run 1, Run 2 present in this region. The MCGOP of Run 3 is on the uptrend after 600 ns of simulation. The result implies that H2O molecules and CH4 molecules form some irregular or incomplete cages in the smectite layer of Run 3, which is same as the result of F4φOP of Run 3. The above results of simulation illustrate that the condition of high ri (Run 3) in the smectite layer can promote the hydrate formation. It is consistent with the previous experimental results in the aqueous solution.52, 53


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The above-mentioned hydrate formation in three systems may be a special case in the process of the hydrate formation. It is inadequate to make an evaluation on the stochastic hydrate formation process in simulation systems. However, the trends of hydrate formation rate in three simulation systems reflect that the molecular diffusion and the ri are the significant influence on hydrate formation in the bulk solution layer and the smectite layer.

4. CONCLUSION In this work, we have performed the molecular-scale hydrate formation process in the systems including a bulk solution layer and a smectite layer through MD simulation. The pure H2O solution (Run 1) and the CH4-H2O homogeneous solution (Run 2 and Run 3) in the initial configuration of the smectite layer are revealed to exert the different influence on the hydrate formation. At the system of pure H2O solution in the smectite layer, the molecular diffusion is due to the density gradient of CH4 molecules between the smectite layer and the bulk solution layer. And then, this molecular diffusion is hinder by the block hydrate formation in the entrance of the smectite layer. Therefore, the cooperativity between molecular diffusion and hydrate formation is involved in the smectite layer of Run 1. At the systems of CH4-H2O homogeneous solution in the smectite layer, the ri affect the molecular diffusion and the hydrate formation not only in the bulk solution layer but also in the smectite layer. It concludes that the high ri can promote the hydrate formation not only in the bulk solution layer but also in the smectite layer. In the smectite layer, the two types of the cage arrangements are present: the semi-cage arrangement and the link-cage arrangement. In the arrangements, the silicon-oxygen ring of the smectite surface could serve as a plane of cage or a site of cage to facilitate the hydrate formation in the smectite layer. Therefore, the silicon-oxygen ring of the smectite surface has the stable


Page 14 of 28

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

effects for the hydrate formation in the smectite nanopore. According to the above work, some experimental phenomenon can be explained by the simulation results. For example, the porous sediment can improve the hydrate formation for the stable effects of the surface structure. The hydrate formation rate in the porous sediment is slower than that in pure water for the gas diffusion and the block of the hydrate in the slit nanopore between smectite basal surfaces. In the next work, we will investigate the effect of the different sediment for hydrate formation. It has further insights into the formation behavior of NGH in sediment.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENTS This work was supported by Key Program of National Natural Science Foundation of China (51736009); Natural Science Foundation of Guangdong Province of China (2017A030313301) ; International S&T Cooperation Program of China (2015DFA61790) ; National Key R&D Program of China (2016YFC0304002, 2017YFC0307306), Science and Technology Apparatus Development Program of the Chinese Academy of Sciences (YZ201619) ; and Frontier Sciences Key Research Program of the Chinese Academy of Sciences (QYZDJ-SSW-JSC033), which are


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

gratefully acknowledged. The authors gratefully acknowledge support from the Guangzhou Branch of the Supercomputing Center of Chinese Academy of Sciences References and the Supercomputer Center of the Computer Network Information Center, Chinese Academy of Sciences.

REFERENCES (1) Wilder, J. W.; Seshadri, K.; Smith, D. H. Modeling Hydrate Formation in Media with Broad Pore Size Distributions. Langmuir 2001, 17 (21), 6729-6735. (2) Linga, P.; Haligva, C.; Nam, S. C.; Ripmeester, J. A.; Englezos, P. Gas Hydrate Formation in a Variable Volume Bed of Silica Sand Particles. Energy Fuels 2009, 23 (11), 5496-5507. (3) Kowalsky, M. B.; Moridis, G. J. Comparison of kinetic and equilibrium reaction models in simulating gas hydrate behavior in porous media. Energ Convers Manage 2007, 48 (6), 1850-1863. (4) Sun, X.; Mohanty, K. K. Kinetic simulation of methane hydrate formation and dissociation in porous media. Chem. Eng. Sci. 2006, 61 (11), 3476-3495. (5) Zang, X.; Lv, Q.; Li, X.; Li, G. Experimental Investigation on Cyclopentane–Methane Hydrate Formation Kinetics in Brine. Energy Fuels 2017, 31 (1), 824-830. (6)

Zhang, Y.; Li, X.S.; Chen, Z.Y.; Li, G.; Wang, Y. Effects of particle and pore sizes on the formation behaviors of methane hydrate in porous silica gels. J. Nat. Gas Sci. Eng. 2016, 35, 1463-1471.

(7) Martos-Villa, R.; Mata, M. P.; Sainz-Díaz, C. I. Characterization of CO2 and mixed methane/CO2 hydrates intercalated in smectites by means of atomistic calculations. J. Mol. Graph. Model. 2014, 49 (0), 80-90. (8) Park, S. H.; Sposito, G. Do montmorillonite surfaces promote methane hydrate formation? Monte Carlo and molecular dynamics simulations. J. Phys. Chem. B 2003, 107 (10), 2281-2290. (9) Seo, Y. J.; Seol, J.; Yeon, S. H.; Koh, D. Y.; Cha, M. J.; Kang, S. P.; Seo, Y. T.; Bahk, J. J.; Lee, J.; Lee, H. Structural, Mineralogical, and Rheological Properties of Methane Hydrates in Smectite Clays. J. Chem. Eng. Data 2009, 54 (4), 1284-1291. (10) Riestenberg, D.; West, O.; Lee, S.; McCallum, S.; Phelps, T. J. Sediment surface effects on methane


Page 16 of 28

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

hydrate formation and dissociation. Mar. Geol. 2003, 198 (1–2), 181-190. (11) Uchida, T.; Takeya, S.; Chuvilin, E. M.; Ohmura, R.; Nagao, J.; Yakushev, V. S.; Istomin, V. A.; Minagawa, H.; Ebinuma, T.; Narita, H. Decomposition of methane hydrates in sand, sandstone, clays and glass beads. J. Geophys. Res.: Solid Earth 2004, 109, B05206. (12) Pirzadeh, P.; Kusalik, P. G. Molecular Insights into Clathrate Hydrate Nucleation at an Ice–Solution Interface. J. Am. Chem. Soc. 2013, 135 (19), 7278-7287. (13) Vatamanu, J.; Kusalik, P. G. Observation of two-step nucleation in methane hydrates. Phys. Chem. Chem. Phys. 2010, 12 (45), 15065-15072. (14) Zhang, Z.; Liu, C. J.; Walsh, M. R.; Guo, G. J. Effects of ensembles on methane hydrate nucleation kinetics. Phys. Chem. Chem. Phys. 2016, 18 (23), 15602-15608. (15) Yagasaki, T.; Matsumoto, M.; Tanaka, H. Formation of Clathrate Hydrates of Water-Soluble Guest Molecules. J. Phys. Chem. C 2016, 120(38), 21512-21521. (16) Bai, D. S.; Chen, G. J.; Zhang, X. R.; Wang, W. C. Microsecond Molecular Dynamics Simulations of the Kinetic Pathways of Gas Hydrate Formation from Solid Surfaces. Langmuir 2011, 27 (10), 59615967. (17) Liang, S.; Rozmanov, D.; Kusalik, P. G. Crystal growth simulations of methane hydrates in the presence of silica surfaces. Phys. Chem. Chem. Phys. 2011, 13(44), 19856-19864. (18) Guo, P.; Pang, Y. K.; Li, L. L.; Tang, B. Molecular dynamics simulation of decomposition and thermal conductivity of methane hydrate in porous media. Chin. Phys. B 2017, 26(7), 073101. (19) Cygan, R. T.; Guggenheim, S.; van Groos, A. F. K. Molecular models for the intercalation of methane hydrate complexes in montmorillonite clay. J. Phys. Chem. B 2004, 108 (39), 15141-15149. (20) Zhou, Q.; Lu, X. C.; Liu, X. D.; Zhang, L. H.; He, H. P.; Zhu, J. X.; Yuan, P. Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations. J. Colloid Interface Sci. 2011, 355, 237-242. (21) Rao, Q.; Xiang, Y.; Leng, Y. Molecular Simulations on the Structure and Dynamics of Water– Methane Fluids between Na-Montmorillonite Clay Surfaces at Elevated Temperature and Pressure. J. Phys. Chem. C 2013, 117, 14061-14069. (22) Yan, K. F.; Li, X. S.; Xu, C. G.; Lv, Q. N.; Ruan, X. K. Molecular dynamics simulation of the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

intercalation behaviors of methane hydrate in montmorillonite. J. Mol. Model. 2014, 20 (6), 2311. (23) Yan, K. F.; Li, X. S.; Chen, Z. Y.; Xia, Z. M.; Xu, C. G.; Zhang, Z. Molecular Dynamics Simulation of the Crystal Nucleation and Growth Behavior of Methane Hydrate in the Presence of the Surface and Nanopores of Porous Sediment. Langmuir 2016, 32 (31), 7975-7984. (24) Karaborni, S.; Smit, B.; Heidug, W.; Urai, J.; van Oort, E. The Swelling of Clays: Molecular Simulations of the Hydration of Montmorillonite. Science 1996, 271, 1102-1104. (25) Titiloye, J. O.; Skipper, N. T. Molecular Dynamics Simulation of Methane in Sodium Montmorillonite Clay Hydrates at Elevated Pressures and Temperatures. Mol. Phys. 2001, 99, 899906. (26) Holmboe, M.; Bourg, I. C. Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature. J. Phys. Chem. C 2014, 118, 1001-1013. (27) Rotenberg, B.; Marry, V.; Malikova, N.; Turq, P. Molecular Simulation of Aqueous Solutions at Clay Surfaces. J. Phys.: Condens. Matter 2010, 22, 284114. (28) Liu, X.; Lu, X.; Meijer, E. J.; Wang, R.; Zhou, H. Atomic-Scale Structures of Interfaces between Phyllosilicate Edges and Water. Geochim. Cosmochim. Acta 2012, 81, 56-68. (29) Kirchner, M. T.; Boese, R.; Billups, W. E.; Norman, L. R. Gas Hydrate Single-Crystal Structure Analyses. J. Am. Chem. Soc. 2004, 126, 9407-9412. (30) Todorov, I. T.; Smith, W.; Trachenko, K.; Dove, M. T. Dl_Poly_3: New Dimensions in Molecular Dynamics Simulations Via Massive Parallelism. J. Mater. Chem. 2006, 16, 1911-1918. (31) Cygan, R. T.; Liang, J. J.; Kalinichev, A. G. Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255-1266. (32) Dashtian, H.; Wang, H.; Sahimi, M. Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics Simulation. J. Phys. Chem. Lett. 2017, 8(14), 3166-3172. (33) Newton, A. G.; Lee, J. Y.; Kwon, K. D. Na-Montmorillonite Edge Structure and Surface Complexes: An Atomistic Perspective. Miner. 2017, 7 (5), 78-96. (34) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926-935.


Page 18 of 28

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(35) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the Opls All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225-11236. (36) Vatamanu, J.; Kusalik, P. G. Molecular Insights into the Heterogeneous Crystal Growth of Si Methane Hydrate. J. Phys. Chem. B 2006, 110, 15896-15904. (37) Bai, D. S.; Zhang, X. R.; Chen, G. J.; Wang, W. C. Replacement Mechanism of Methane Hydrate with Carbon Dioxide from Microsecond Molecular Dynamics Simulations. Energ Environ. Sci. 2012, 5, 7033-7041. (38) Tung, Y. T.; Chen, L. J.; Chen, Y. P.; Lin, S. T. The Growth of Structure I Methane Hydrate from Molecular Dynamics Simulations. J. Phys. Chem. B 2010, 114, 10804-10813. (39) Cao, Z.; Tester, J. W.; Sparks, K. A.; Trout, B. L. Molecular Computations Using Robust Hydrocarbon−Water Potentials for Predicting Gas Hydrate Phase Equilibria. J. Phys. Chem. B 2001, 105, 10950-10960. (40) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald - an N.Log(N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98(12), 10089-10092. (41) BÁEz, L. A.; Clancy, P. Computer Simulation of the Crystal Growth and Dissolution of Natural Gas Hydratesa. Ann. N. Y. Acad. Sci. 1994, 715, 177-186. (42) Rodger, P. M.; Forester, T. R.; Smith, W. Simulations of the Methane Hydrate Methane Gas Interface near Hydrate Forming Conditions. Fluid Phase Equilib. 1996, 116, 326-332. (43) Barnes, B. C.; Beckham, G. T.; Wu, D. T.; Sum, A. K. Two-Component Order Parameter for Quantifying Clathrate Hydrate Nucleation and Growth. J. Chem. Phys. 2014, 140. (44) Sarupria, S.; Debenedetti, P. G. Homogeneous Nucleation of Methane Hydrate in Microsecond Molecular Dynamics Simulations. J. Phys. Chem. Lett. 2012, 3, 2942-2947. (45) Tung, Y. T.; Chen, L. J.; Chen, Y. P.; Lin, S. T. Growth of Structure I Carbon Dioxide Hydrate from Molecular Dynamics Simulations. J. Phys. Chem. C 2011, 115, 7504-7515. (46) Zhang, Z. C.; Guo, G. J. The effects of ice on methane hydrate nucleation: a microcanonical molecular dynamics study. Phys. Chem. Chem. Phys. 2017, 19 (29), 19496-19505. (47) Walsh, M. R.; Beckham, G. T.; Koh, C. A.; Sloan, E. D.; Wu, D. T.; Sum, A. K. Methane Hydrate


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nucleation Rates from Molecular Dynamics Simulations: Effects of Aqueous Methane Concentration, Interfacial Curvature, and System Size. J. Phys. Chem. C 2011, 115 (43), 21241-21248. (48) Chialvo, A. A.; Houssa, M.; Cummings, P. T. Molecular Dynamics Study of the Structure and Thermophysical Properties of Model Si Clathrate Hydrates. J. Phys. Chem. B 2002, 106, 442-451. (49) Geng, C. Y.; Wen, H.; Zhou, H. Molecular Simulation of the Potential of Methane Reoccupation During the Replacement of Methane Hydrate by Co 2 . J. Phys. Chem. A 2009, 113, 5463-5469. (50) Koh, C. A.; Wisbey, R. P.; Wu, X. P.; Westacott, R. E.; Soper, A. K. Water Ordering around Methane During Hydrate Formation. J. Chem. Phys. 2000, 113, 6390-6397. (51) Moon, C.; Hawtin, R. W.; Rodger, P. M. Nucleation and control of clathrate hydrates: insights from simulation. Faraday Discuss 2007, 136, 367-382. (52) Natarajan, V.; Bishnoi, P. R.; Kalogerakis, N. Induction phenomena in gas hydrate nucleation. Chem. Eng. Sci. 1994, 49, (13), 2075-2087. (53) Kashchiev, D.; Firoozabadi, A. Induction time in crystallization of gas hydrates. J. Cryst. Growth 2003, 250(3-4), 499-515.


Page 20 of 28

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table Caption Table 1. The physical parameters of the simulation systems. Figure Captions Figure 1. Molecular configuration at the initial simulation at x-z plane (a) Run 1, (b) Run 2, (c) Run 3. Red and white stick indicates H2O molecules, yellow sphere indicates CH4 molecules, yellow and purple stick indicates montmorillonite molecule, purple blue sphere indicates Na+ ions, the blue dashed line indicates the H-bond. Figure 2. Snapshots of the hydrate formation simulation at 1µs at x-z plane. Figure 3. RDFs of the simulation systems in the smectite layer (a) gOwtr-Owtr(r), (b) gC-C(r). Figure 4. The cage configurations of Run 3 at 1µs (a) the partial integrated cages in the bulk solution layer and in the smectite layer, (b) the cage arrangements in the smectitie layer. Figure 5. RDFs of Run 3 in the smectite layer. Figure 6. F4φOPs for the trajectory of the simulations (a) in the bulk solution layer and (b) in the smectite layer. Figure 7. The z-profile for the F4φOPs for the simulation systems. Figure 8. MCGOPs for the trajectory of the simulations (a) in the bulk solution layer and (b) in the smectite layer.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

Table 1. The physical parameters of the simulation systems. 0 ns

simulation

1 µs

the bulk solution the smectite layer

the bulk solution the smectite layer

layer

layer

H2O CH4

ri

H2O CH4

ri

H2O CH4

Run 1

384

0

0

500

87

0.174

371

17

0.046

513

70

0.136

Run 2

338

32

0.095

546

55

0.101

359

26

0.072

525

61

0.116

Run 3

338

32

0.095

546

87

0.159

368

36

0.098

516

83

0.161


ri

H2O CH4

ri

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure

Figure 1. Molecular configuration at the initial simulation at x-z plane (a) Run 1, (b) Run 2, (c) Run 3. Red and white stick indicates H2O molecules, yellow sphere indicates CH4 molecules, yellow and purple stick indicates montmorillonite molecule, purple blue sphere indicates Na+ ions, the blue dashed line indicates the H-bond.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Snapshots of the hydrate formation simulation at 1µs at x-z plane.


Page 24 of 28

Page 25 of 28

7 6

(b)

10

In the smectite layer

Run 1 Run 2 Run 3

9 8

5

7

4

6

gC-C(r)

(a)

gOwtr-Owtr(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3


Run 1 Run 2 Run 3

5 4 3

2

2

1 1

0 0.0

0.2

0.4

0.6

0.8

1.0

0 0.0

r (nm)

0.2

0.4

0.6

0.8

1.0

r (nm)

Figure 3. RDFs of the simulation systems in the smectite layer (a) gOwtr-Owtr(r), (b) gC-C(r).


Energy & Fuels

Figure 4. The cage configurations of Run 3 at 1µs (a) the partial integrated cages in the bulk solution layer and in the smectite layer, (b) the cage arrangements in the smectitie layer.

7

Owtr-Owtr Osmectite-Owtr

6

C-Owtr

5

C-Osmectite

g(r)

4 3 2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

r (nm)

Figure 5. RDFs of Run 3 in the smectite layer.

(a)

(b)

0.7

0.4

Run 1 Run 2 Run 3

In the bulk solution layer 0.6 0.3


0.5

F4ϕ

0.4

F4ϕ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

0.2

0.3 0.2

Run 1 Run 2 Run 3

0.1

0.1

0.0

0.0 0

100 200 300 400 500 600 700 800 900 1000

0

100 200 300 400 500 600 700 800 900 1000

Time (ns)

Time (ns)

Figure 6. F4φOPs for the trajectory of the simulations (a) in the bulk solution layer and (b) in the smectite layer.


1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1

the smectite layer

the bulk solution layer

0

1

2

3

4

Run 1 Run 2 Run 3

5

6

7

Z-coordinate (nm)

Figure 7. The z-profile for the F4φOPs for the simulation systems.

(a)

(b)

70

30

Run 1 Run 2 Run 3

In the bulk solution layer

Run 1 Run 2 Run 3

60

25


50 20

MCGOP

MCGOP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

F4ϕ

Page 27 of 28

40 30

15

10

20 5

10 0

0

0

200

400

600

800

1000

0

Time (ns)

200

400

600

800

1000

Time (ns)

Figure 8. MCGOPs for the trajectory of the simulations (a) in the bulk solution layer and (b) in the smectite layer.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic


Page 28 of 28

The Formation of CH4 Hydrate in the Slit Nanopore between the

Recommend Documents