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Aug 10, 2015 - about 290 K at 10 MPa. The growth rate is found to be determined by two competing factors: (1) the adsorption of CH4 at the solid−liq...
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Molecular Dynamics Study on the Growth Mechanism of Methane plus Tetrahydrofuran Mixed Hydrates Jyun-Yi Wu, Li-Jen Chen, Yan-Ping Chen, and Shiang-Tai Lin* Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Molecular dynamics (MD) simulations are performed to analyze the dominating factors for the growth of CH4 + THF mixed hydrates, and the results are compared with the growth of single guest CH4 and THF hydrates. While CH4 hydrate has a type I crystalline structure, the presence of THF in the aqueous phase results in the growth the type II structure hydrate. Compared to THF hydrates, the presence of CH4 in the system enhances the dissociation temperature. The growth rate of CH4 + THF mixed exhibits a maximum value at about 290 K at 10 MPa. The growth rate is found to be determined by two competing factors: (1) the adsorption of CH4 at the solid−liquid interface, which is enhanced with decreasing temperature, and (2) the migration of THF to the proper site at the interface, which is enhanced with increasing temperature. Above 290 K, which is about 10 K higher than the dissociation temperature of pure THF hydrate, the growth of cage can proceed only when a sufficient amount of CH4 is adsorbed at the interface. The growth rate is dominated by the uptake of CH4 at the interface, as in the case of pure CH4 hydrate. Below 290 K, the growth is not much affected by the presence of CH4. Instead, the growth rate is determined by the rearrangement of THF molecules at the interface, as in the case of pure THF hydrate. as much as 18 K12,14 when there is 5.56 mol % of THF in the aqueous phase. In fact, the presence of THF results in a different crystalline structure. Pure methane hydrates are of type I structure, where methane molecules stay in both the small (pentagonal dodecahedron, 512) and large (hexagonal truncated trapezohedron, 51262) cages created by the hydrogen-bonded water molecules. The presence of THF would result in the formation of type II structure, with methane molecules occupying the small 512 cages and the THF occupying the larger (hexadecahedron, 51264) cages. Type I structure (sI) contains two 512 and six 51262 cages, and type II structure (sII) consists of sixteen 512 and eight 51264 cages. Although the occupancy of methane increases with pressure, THF almost always occupies all the 51264 cages in the type II structure.16,17 Therefore, the loading of methane in the methane + THF mixed hydrate is significantly less than that in pure methane hydrate.16 While THF is a powerful thermodynamic promotor of gas hydrates, the growth rate of CH4 + THF hydrates is found to be slower than that of CH4 hydrates under the same subcooling temperature.18 However, the fundamental reason for the slower growth kinetics with the presence of THF is not clearly known. MD simulations have been used to provide molecular insights into many important issues of gas hydrates, including nucleation,19−24 guest replacement,25−27 growth/melting mechanism,28−31 and structure changes.32,33 It has been shown that

1. INTRODUCTION Clathrate hydrates are a class of crystalline solids consisting of small guest molecules (such as methane or CO2) encapsulated in the cage spaces formed by hydrogen-bonded water molecules. Naturally occurring clathrate hydrates, mostly methane hydrates, are found in permafrost region and sea floors. The amount of methane trapped in the form of hydrate is so abundant such that it is considered as a potential source of energy.1−4 Furthermore, due to the characteristic of high gas density at relatively high temperature and low pressure, gas hydrates are also considered as a good means for transport and storage of large quantities of gas, such as natural gas,5,6 carbon dioxide,5 and hydrogens.7−11 The knowledge of thermodynamic stability of clathrate hydrates and how their stability limit can be enhanced or reduced is important for developing processes involving them. One possible means to change the dissociation temperature of gas hydrates is to control the system pressure. For example, the dissociation temperature of methane hydrate is 273 K at 25 atm and can be enhanced with increasing pressure.12 An alternative way to alter the stability of gas hydrate is to introduce additives to the aqueous solution. For example, alcohols (e.g., methanol, ethanol) are known to be effective inhibitors that reduce the dissociation temperature under constant pressure. On the other hand, cyclic ethers (e.g., tetrahydropyran, cyclobutanone, methylcyclohexane) are promotors that increase the dissociation temperature of gas hydrates.13 Tetrahydrofuran (THF) is known to be a very effective thermodynamic promoter of gas hydrates.10,12,14,15 The formation temperature of methane hydrate can be enhanced by © XXXX American Chemical Society

Received: June 5, 2015 Revised: August 6, 2015

A

DOI: 10.1021/acs.jpcc.5b05393 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Initial structure of the gas−liquid−hydrate three-phase model of CH4 + THF hydrate. This model corresponds to a liquid phase THF = xTHF concentration of xTHF L H . The system contains 112 THF 416 CH4 and 1904 water molecules.

target temperature. After these pre-equilibration steps (time zero point in our analysis), long (up to 1.7 μs) NPT simulations were then performed for analysis. The leapfrog algorithm41 is used with a integration time step of 1 fs. The cutoff radius for both van der Waals and Coulomb interactions is 0.95 nm. Long-range Coulomb interactions are determined using the particle-mesh Ewald (PME)42 method. The Nosé−Hoover thermostat43 with tau_t = 1 ps is used for temperature control and the Parrinello−Rahman44 with tau_P = 10 ps for the pressure control. Anisotropic pressure control (one for the growing direction and one for the other two directions) is adopted for the liquid−hydrate−liquid−gas three-phase model. Isotropic pressure control is adopted for the simulation without hydrate phase (for the diffusivity and solubility tests). 2.3. Force Field. The TIP4P-Ice force field45 is choose in this study for H2O molecules. This force field is known to reproduce the phase boundary between liquid water and ice, the density of both liquid water and ice, the melting enthalpy, and the slope of the coexistence curve. For CH4 molecules, the OPLS-AA model46 is employed. The geometric mean is used for the offdiagonal terms for the Lennard-Jones (LJ) potential between water and CH4. For THF molecules, model 7 of Girard et al.47 was slightly modified (see Table 1) to better reproduce the

the solubility of CH4 in water, the transport of CH4 to the hydrate/liquid interface, and the uptake of methane on the interface are the key factors determining the growth rate of methane hydrates.28 For pure THF hydrates, the transport of THF to the interface and the migration of interfacial THF are the dominating growth factors.34,35 Recent work of Erfan-Niya et al.36 investigated the bulk properties of CH4 + THF mixed hydrates. However, to the best of our knowledge, there has not been any report on the kinetics and growth mechanism of CH4 + THF mixed hydrates. Therefore, the aim of the present work is to calculate the growth rates and analyze the growth mechanism of CH4 + THF hydrate by MD simulations. Our results help to provide insights into the kinetic inhibition effect of THF during the growth of methane hydrates.

2. SIMULATION DETAILS 2.1. Molecular Model. The liquid−hydrate−liquid−gas (LH-L-G) three-phase model, as shown in Figure 1, was created using Materials Studio37 for the simulation of dissociation condition and growth rate of CH4 + THF hydrate. The √2 × √2 × 2 sII hydrate crystal with (100) crystal face has each of its large cages filled with one THF molecule and each of small cages filled with one CH4 molecule (a total of 32 THF and 64 CH4). The (100) crystal face was chosen because it was found the fastest growth face of THF hydrate in a previous study.35 For the ease of future analysis, we define a unit hydrate layer (HL) as the repeating unit in the hydrate phase (two HLs in the hydrate phase in Figure 1). Each HL can be further divided into four equally sized sublayers (SL) along the z-direction.34 Before filling in guest molecules, the empty sII lattice was annealed by heating and cooling with the position of oxygen atoms fixed. This allows for the water molecules to rotate and reconstruct the hydrogen bond network such that a structure of zero net dipole moment can be obtained. The liquid phase initially contains only water and THF (with the ratio of THF:water being the same as that in the hydrate phase) and free of any CH4. It is expected that both the CH4 diffusion and the THF rearrangement at hydrate interface (which has been shown to be important at such high THF concentration condition34) will affect the growth of CH4 + THF hydrate in our simulation. 2.2. Molecular Dynamics (MD) Simulation. The MD simulations were performed using GROMACS 4.5.38−40 The initial structure was first energy minimized to eliminate any bad contacts. A 20 ps MD simulation was then conducted at 200 K under constant volume (NVT) to further relax any extra stress in the system. A subsequent temperature rising process was then performed to increase the temperature the desired value at a rate of 0.5 K/ps, followed by an additional 100 ps simulation at the

Table 1. Density and Heat of Vaporization of THF at 300 K and 1 atm Obtained Using Different Scaling Factors for the LJ 12−6 Potential Parameters of Girad

a

Girard 7 modified Girard 7 exptl data

fσa

fε a

1 1.003

1 0.933

density (kg/m3)

Hvap (kJ/mol)

904 880 880b

34.32 31.92 31.803 ± 0.012c

a

Factors for modifying the LJ 12−6 parameters from Girad’s work47 σnew = fσ × σGirad and εnew = fε × εGirad with i = C, H, or O. σGirad = i i i i C Girad = 0.350, σ = 0.190 (nm), εGirad = 0.190, εGirad = 0.360, 0.385, σGirad O H C O = 0.150 (kJ/mol). bAt 1 atm and 298.15 K.48 cAt 0.25 atm and εGirad H 300.80 K.49

density and heat of vaporization of pure THF liquid. The offdiagonal LJ potential parameters between THF and water were taken from previous study.34 The solubility of THF in water and the dissociation temperature of THF hydrate from these settings are in good agreement with experiment. (Note that the solubility of THF was underestimated and the dissociation temperature of THF hydrate was slightly overestimated if the geometric mean was used to determine the off-diagonal LJ parameters.) It should be noted that no data of the mixed CH4 + THF hydrates were B

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The Journal of Physical Chemistry C used for the parameter optimization. The off-diagonal LJ parameters are summarized in Table 2. Table 2. Off-Diagonal LJ 12−6 Potential Parameters between THF and Water CTHF−OH2O

OTHF−OH2O

force field

ε (kcal/mol)

σ (Å)

ε (kcal/mol)

σ (Å)

geometric mean modified34

0.0944 0.2547

3.496 3.430

0.1301 0.1509

3.334 3.163

2.4. Growth/Melting Rate. The growth rate, r, of hydrate is calculated from the number of water molecules added to the hydrate phase from the 100 to 300 ns of the NPT simulation. The data of initial 100 ns was taken as the time needed for the system to reach steady growth. number of water molecules increased THL r= 200 ns WHL

Figure 2. Dissociation condition of CH4 hydrates (green triangles), THF hydrates (red diamonds), and CH4 + THF hydrates (blue squares) from MD simulations and the experiment (lines).12,53,54

Nonetheless, the promoting effect of THF for CH4 is correctly predicted. 3.2. Growth Rate. The growth rates of CH4 + THF mixed hydrate are determined at temperatures below the dissociation condition (312.5 K) at 10 MPa. Figure 3 illustrates the time evolution of the hydrate phase thickness (in terms of hydrate layers, HL) from three independent sets of simulations at three temperatures. In general, the thickness increases with time, but there are ups and downs during the period of our growth simulation. The cause of pauses and temporary dissociation will be discussed in the next section. The slope of the curves in Figure 3d (average of the independent runs) gives the growth rates. The data of initial 100 ns in our simulation are discarded (see eq 1) to avoid the effect from the initial relaxation processes. (There is a clear change of slope in Figure 3d in the initial 5−10 ns as a result of the diffusion of methane to the aqueous phase.) Figure 4 summarizes the growth rate of CH4 + THF hydrate with different subcooling temperatures at 10 MPa. The growth rate exhibits a maximum value at 290 K (a subcooling temperature of 22.5 K). Furthermore, the growth rate appears to be independent of temperature below 280 K. Table 3 shows the growth rate of CH4, THF, and CH4 + THF hydrate at different temperature and pressure. Note that under the same degree of subcooling (e.g., 7.5 K at 10 MPa) the growth rate of CH4 hydrates, 4.08 ± 0.20 mm/s (275 K), is 3 times greater than that of CH4 + THF hydrate, 1.20 ± 1.82 mm/s (305 K). Such a kinetic inhibition effect of THF to CH4 hydrate was also observed in a previous experimental study.18 However, it is noteworthy that the comparison of growth rate between CH4 hydrate and CH4 + THF hydrate is not meaningful because they are of different crystalline structures. A more sensible comparison is between THF hydrate and CH4 + THF hydrate, where it is found that CH4, a thermodynamic promoter of THF hydrate, is a kinetic inhibitor below the dissociation temperature of THF hydrate and a kinetic promotor near and above the dissociation temperature of THF hydrate. It is noteworthy that factors such as the size of the molecular model and the thickness of the aqueous layer can have an influence on the value of the growth rate. The strength of MD simulations is not to provide the quantitative value of growth rate but to provide insights, as will be discussed in the following section, into how the different factors (in our case the temperatures) affect the growth rate. 3.3. Growth Mechanism. In Figure 3, we observe that the growth of CH4 + THF hydrate is not a steady process. For example, in 10 MPa−275 K−run 3, the growth pauses at 20−60

(1)

where THL and WHL are the thickness and number of water molecules in one hydrate layer (HL). The values of these two properties depends on the structure of the hydrates. For sII hydrates (THF and CH4 + THF), THL = 17.2 Å and WHL = 272. For sI hydrates (CH4), THL = 11.9 Å and WHL = 184. For better statistics, the growth rates are averaged from the results of three independent runs (generated using different random seeds for the initial velocity). The method for determining the number of water used in hydrate is the same as the previous study and shown in Figures S1−S3 of the Supporting Information.34

3. RESULTS AND DISCUSSION 3.1. Validation of the Force Field. The thermodynamic properties such as the solubility of guests in water, dissociation temperatures of hydrates, and the thermodynamic promotion effect of THF for CH4 hydrates can serve as a validation of the force field used in this study. The solubility is determined using NPT simulation for a two-phase (gas and liquid) model (as in ref 34). The solubility of methane in water is found to be 0.128 mol/ kg water (or 434 water per CH4) at 10 MPa and 305 K, which is in good agreement with the experiment,50 0.111 mol/kg water (or 500 water per CH4) at 10.22 MPa and 303.2 K. THF and water are completely miscible at ambient conditions. However, as shown in Figure S4a, the use of geometric mean for the offdiagonal LJ terms between THF and water (see Table 2) results in partially miscible two-liquid phases. This implies that the interactions between THF and water are too weak. The modified off-diagonal parameters (Table 2) increases the attractive interactions between THF and water and thus results in a completely miscible mixture (Figure S4b) as observed in experiment.51 The dissociation temperature (Td) of hydrates is determined by performing several NPT simulations with different temperatures under the same pressure. If the temperature is above Td, the hydrate phase would melt and the potential energy would increase. If the temperature is below Td, then the opposite would happen.27−30,34,52 Figure 2 compares the dissociation temperatures as a function of pressure for CH4, THF, and CH4 + THF hydrates from our simulation (open symbols) and experiment (lines). Both the dissociation temperatures of THF hydrate and CH4 hydrate are well reproduced. The dissociation temperatures of CH4 + THF hydrates are slightly overestimated by 5−10 K. C

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Figure 3. Time evolution (growth) of CH4 + THF hydrate layers at 10 MPa and 275 K (a), 290 K (b), and 305 K (c). Shown in each figure are results of three independent runs. The average values are compared in (d). The dissociation temperature at 10 MPa is 312.5 K.

interface are notably different. The difference can be understood from the distribution of guest molecules at the interface. Figure 6 illustrates the cross section of the sublayer 1 (SL1) on the leftand right-hand sides of the initial hydrate crystal at 30 ns and 1 μs. At 30 ns (near the beginning of simulation), the sublayer 1 on both sides are both solid−liquid interfaces. Both the THF and CH4 molecules adsorbed on the small and large cage sites (see Figure 6a,b). However, all the large cage sites are occupied by THF molecules, and some of small cage sites are filled by CH4 molecules after the growth finishes, i.e., Figure 6c,d. The rearrangement of guest molecules to the proper sites at the interface is necessary for the successful growth of a SL. To investigate the reason for intermittency in the growth process in more detail, the number of guest molecules (THF and CH4) absorbed to different sublayers (SL) during the course of simulation is analyzed. Figure 7 shows the number of guest molecules (THF and CH4) absorbed to different sublayers (SL) at 297.5 K during the course of simulation. It should be noted that each SL has four large cage (51264) sites and eight small cage (512) sites (see Figure 6). As the growth process begins, the THF molecules are adsorbed on both large and small cage sites, and thus the total number of THF quickly exceeds 4 (see SL1 before 200 ns (Figure 7a), SL2 from 200 to 400 ns (Figure 7b), and SL3 from 400 to 700 ns (Figure 7c)). To proceed with the growth, the excess THF molecules, in particular those adsorbed on the small cage sites, must be desorbed. At high temperatures (e.g., above dissociation temperature of pure THF hydrate, 276 K at 10 MPa), the THF molecules can escape from the interfacial cage sites more easily. As can be seen in SL3 from 400 to 700 ns, the number of THF molecules in both large and small cage sites changes quickly. Notice that the growth of SL3 (from 390 to 820 ns) is slower than that of SL1 (from 0 to 180 ns) and SL2 (from 180 to 390 ns). One obvious reason is the number of CH4 in SL3 remained at a low value (fluctuating between 0 and 1 most of the time) compared to those in other SLs. Before 700 ns, the number of CH4 molecules at the interface is always equal to or smaller than 2 (Figure 8g). After 700 ns, the number reaches 4 or 5 and the growth of the SL quickly finishes. In other words, the adsorption of enough CH4 at the interface is important at this temperature. This is sensible because without CH4 the THF hydrate would not from as the simulation temperature is already above the dissociation temperature of THF hydrate. In other

Figure 4. Growth rate of CH4 + THF hydrate as a function of temperature at 10 MPa.

Table 3. Growth Rate of CH4, THF, and CH4 + THF Hydrates from MD Simulations system

P (MPa)

T (K)

growth rate (mm/s)

10 10 10 0.1 10 10 10 10 10 10 10 10 10

260 267.5 275 270 262.5 262.5 270 275 280 285 290 297.5 305

3.56 3.62 ± 0.61 4.08 ± 0.20 8.15 ± 1.57 7.37 ± 0.80 3.91 ± 1.48 3.13 ± 2.24 2.83 ± 3.06 2.92 ± 0.81 2.93 ± 2.02 6.10 ± 2.89 2.28 ± 2.10 1.20 ± 1.84

CH4 hydrate

THF hydrate CH4 + THF hydrate

ns, 70−145 s, 180−220 ns, and 240−270 ns. The duration of pauses may take as long time as 100 ns (e.g., 70−300 ns in 10 MPa−275 K−run 2 or 20−300 ns in 10 MPa−305 K−run 3). These pauses result in a slower value and higher uncertainty in the calculated growth rate. Figure 5 illustrates an example of the growth of CH4 + THF hydrate at 10 M Pa and 297.5 K. After 1 μs (Figure 5b) the thickness of the CH4 + THF hydrate increased to 4 hydrate layers. The growth rates of the left interface and right D

DOI: 10.1021/acs.jpcc.5b05393 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. Snapshots of CH4 + THF hydrate growth at 10 MPa and 297.5 K at 30 ns (a) and 1 μs (b). The location of the solid−liquid interface at time 0 ns is indicated by bold dashed lines. The blue dashed lines are used to illustrate the ID of sublayers (SL) for discussion in the main text.

Figure 6. Cross section of left and right (b, d) sublayer one (SL1) from CH4 + THF hydrate growth at 30 ns (a: left surface, b: right surface) and 1 μs (c: left surface, d: right surface).

methane is enhanced.28 However, the rearrangement of THF at the interface is easier at high temperatures because of enhanced molecular mobility.34 The different temperature dependence of these two competing factors results in a maximum growth rate at a temperature (about 290 K) that is slightly higher than the dissociation temperature of pure THF hydrate (276 K). It is reasonable to presume that between 276 and 290 K the growth is dominated by the rearrangement of THF. The temperature dependence of growth rate also affects the occupancy of CH4 given in Table 4. At 10 MPa and 290 K the CH4 occupancy of small cages reaches a minimum value of 0.269. Above 290 K, the occupancy of CH4 must be higher because the hydrate interface would not be stable otherwise. Below 290 K, the low growth rate (limited by THF rearrangement) allowed enough time for more CH4 adsorbing on the interface. The occupancy of the large cages is close to unity in all cases. This is because that the THF hydrates are not stable without the large cages being occupied.34 The high concentration of THF results

words, the occupancy of CH4 in the small cages is important to stabilize the hydrate structure at high temperatures. Figure 8 shows the situation at 262.5 K, which is below the dissociation temperature of pure THF hydrate. In this case, the presence of CH4 molecules is not indispensable for the growth of structure II hydrate. As can be seen in Figure 8e,f, the growth of the hydrate layer can proceed even without any CH4 at the interface. On the other hand, the rearrangement of THF at the interface becomes more difficult because of the lowered mobility of water and THF. (The result was shown in the previous study; the ideal liquid THF concentration in THF hydrate growth is higher in higher temperature.34) For example, the growth of SL2 (from 80 to 150 ns) does not finish until all the THF molecules leave the small cage sites. The rate-determining process thus changes to the rearrangement of guest molecules at the interface at low temperatures. It is interesting to note that the adsorption of CH4 at the interface is favored at lower temperatures as the solubility of E

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Figure 7. Number of THF (a, b, and c for layers 1, 2, and 3, respectively) and CH4 (d, e, and f for layers 1, 2, and 3, respectively) molecules located at the aqueous phase near the left-hand-side interface. The system is at 10 MPa and 297.5 K.

Figure 8. Number of THF (a, b, and c for layers 1, 2, and 3, respectively) and CH4 (d, e, and f for layers 1, 2, and 3, respectively) molecules located at the aqueous phase near the left-hand-side interface. The system is at 10 MPa and 262.5 K.

4. CONCLUSION

in fast adsorption of THF to the large cage sites, and thus the growth rate is dominated by slower processes, adsorption of CH4 at high temperatures and THF desorption/migration at low temperatures. Also shown in Table 4 is the occupancy reported from NMR study at 4 MPa and 292 K.16 The occupancy from our simulation at 5 MPa and 290 K agrees quite well with the experiment, which again confirms the reliability of our simulation.

The growth mechanism of methane and tetrahydrofuran clathrate hydrates was investigated by MD simulations. The force field used provides reasonable accuracy for a variety of relevant thermodynamic properties, including the melting point of ice, dissociation temperature of THF, CH4, and CH4 + THF hydrates, and solubility of CH4 and THF in water. The kinetic inhibition effect of THF in CH4 hydrate was also observed. Our simulations show that the growth of CH4 + THF mixed hydrate F

DOI: 10.1021/acs.jpcc.5b05393 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 4. Occupancy of Guests in the Newly Grown CH4 + THF Hydrate Layers 512 source

P (MPa)

T (K)

CH4

NMR16 MD

4 5 10 10 10 10 10 10 10 10

292 290 262.5 270 275 280 285 290 297.5 305

0.3701 0.4262 0.4419 0.5900 0.4175 0.4312 0.3654 0.2937 0.4224 0.6141

THF 0 0 0 0 0 0 0 0 0

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05393. Procedure of identifying the water structures in hydrates and the results of THF solubility in water (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.-T.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the financial support of this research from the Ministry of Economic Affairs (103-5226904000-03-03) and the Ministry of Science and Technology (MOST 103-3113M-002-006) of Taiwan.

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CH4

THF

CH4 + THF

0.0227 0 0.0128 0.0198 0.0025 0.0003 0.0519 0 0

0.9989 0.9769 0.9694 0.9872 0.9802 0.9662 0.9997 0.9481 1 1

0.9989 0.9996 0.9694 1.0000 1.0000 0.9687 1.0000 1.0000 1 1

(4) Kvenvolden, K. A. Gas Hydrates - Geological Perspective and Global Change. Rev. Geophys. 1993, 31, 173−187. (5) Chatti, I.; Delahaye, A.; Fournaison, L.; Petitet, J. P. Benefits and Drawbacks of Clathrate Hydrates: A Review of Their Areas of Interest. Energy Convers. Manage. 2005, 46, 1333−1343. (6) Gbaruko, B. C.; Igwe, J. C.; Gbaruko, P. N.; Nwokeoma, R. C. Gas Hydrates and Clathrates: Flow Assurance, Environmental and Economic Perspectives and the Nigerian Liquified Natural Gas Project. J. Pet. Sci. Eng. 2007, 56, 192−198. (7) Chapoy, A.; Anderson, R.; Tohidi, B. Low-Pressure Molecular Hydrogen Storage in Semi-Clathrate Hydrates of Quaternary Ammonium Compounds. J. Am. Chem. Soc. 2007, 129, 746−747. (8) Kim, D.-Y.; Park, Y.; Lee, H. Tuning Clathrate Hydrates: Application to Hydrogen Storage. Catal. Today 2007, 120, 257−261. (9) Prasad, P. S. R.; Sowjanya, Y.; Shiva Prasad, K. Micro-Raman Investigations of Mixed Gas Hydrates. Vib. Spectrosc. 2009, 50, 319− 323. (10) Sugahara, T.; Haag, J. C.; Prasad, P. S. R.; Warntjes, A. A.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Increasing Hydrogen Storage Capacity Using Tetrahydrofuran. J. Am. Chem. Soc. 2009, 131, 14616−14617. (11) Ogata, K.; Tsuda, T.; Amano, S.; Hashimoto, S.; Sugahara, T.; Ohgaki, K. Hydrogen Storage in Trimethylamine Hydrate: Thermodynamic Stability and Hydrogen Storage Capacity of Hydrogen Plus Trimethylamine Mixed Semi-Clathrate Hydrate. Chem. Eng. Sci. 2010, 65, 1616−1620. (12) de Deugd, R. M.; Jager, M. D.; de Swaan Arons, J. Mixed Hydrates of Methane and Water-Soluble Hydrocarbons Modeling of Empirical Results. AIChE J. 2001, 47, 693−704. (13) Mooijer-van den Heuvel, M. M.; Peters, C. J.; de Swaan Arons, J. Influence of Water-Insoluble Organic Components on the Gas Hydrate Equilibrium Conditions of Methane. Fluid Phase Equilib. 2000, 172, 73− 91. (14) Chari, V. D.; Sharma, D. V. S. G. K.; Prasad, P. S. R. Methane Hydrate Phase Stability with Lower Mole Fractions of Tetrahydrofuran (Thf) and Tert-Butylamine (T-Bunh2). Fluid Phase Equilib. 2012, 315, 126−130. (15) Strobel, T. A.; Koh, C. A.; Sloan, E. D. Thermodynamic Predictions of Various Tetrahydrofuran and Hydrogen Clathrate Hydrates. Fluid Phase Equilib. 2009, 280, 61−67. (16) Seo, Y. T.; Lee, H. C-13 Nmr Analysis and Gas Uptake Measurements of Pure and Mixed Gas Hydrates: Development of Natural Gas Transport and Storage Method Using Gas Hydrate. Korean J. Chem. Eng. 2003, 20, 1085−1091. (17) Yoon, J.-H. A Theoretical Prediction of Cage Occupancy and Heat of Dissociation of Thf-Ch4 Hydrate. Korean J. Chem. Eng. 2012, 29, 1670−1673. (18) Lim, S.-H.; Riffat, S. B.; Park, S.-S.; Oh, S.-J.; Chun, W.; Kim, N.-J. Enhancement of Methane Hydrate Formation Using a Mixture of Tetrahydrofuran and Oxidized Multi-Wall Carbon Nanotubes. Int. J. Energy Res. 2014, 38, 374−379. (19) Jacobson, L. C.; Hujo, W.; Molinero, V. Amorphous Precursors in the Nucleation of Clathrate Hydrates. J. Am. Chem. Soc. 2010, 132, 11806−11811.

is dominated by the adsorption of CH4 to the growing interface and the migration and rearrangement of THF at the interface. At temperature higher than the dissociation temperature of pure THF hydrate, the presence of CH4 provides the necessary lattice framework in order to stabilize the crystalline structure. Therefore, the growth rate is dominated by the rate of CH4 accumulation at interface at high temperatures. At temperature below the melting point of THF hydrate, the growth can proceed without the aid of CH4. Under such circumstances, the growth rate is dominated by the escape of THF from the incorrect sites on the interface, which is also the dominating factor for the growth of pure THF hydrate observed previously.34 The different temperature dependence of these two phenomena results in a competing effect. The adsorption of CH4 is enhanced as the temperature is decreased, but the migration of THF is enhanced with increasing temperature. As a result, the growth rate of CH4 + THF hydrate exhibits a maximum value at around 290 K at 10 MPa.

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DOI: 10.1021/acs.jpcc.5b05393 J. Phys. Chem. C XXXX, XXX, XXX−XXX