Introduction

As the global demand for energy continues to rise, finding efficient and environmentally friendly methods for energy storage and transportation becomes increasingly crucial. Solidified natural gas (SNG) storage using clathrate hydrates has emerged as a promising solution, offering a novel approach to store and transport natural gas in a condensed form [1,2,3,4,5]. Clathrate hydrates are unique crystalline structures formed when water molecules encapsulate guest molecules, typically gases, within their cage-like framework [6, 7]. These structures resemble ice and exhibit remarkable properties, including high gas storage capacity, low density, and enhanced stability under appropriate temperature and pressure conditions [6, 7]. The gas molecules trapped within clathrate hydrates are efficiently sequestered, resulting in a denser and more manageable storage medium for natural gas [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25].

The exploration of clathrate hydrates for gas storage dates back several decades, primarily focusing on methane hydrates because of their abundance in nature and potential as an unconventional energy resource [6, 7]. However, recent research has broadened the scope by including other guest molecules like tetrahydrofuran (THF) and cyclopentane (CP), which possess unique properties and hold promise for SNG storage applications [24, 25]. THF and CP are commonly used as thermodynamic hydrate promoters and have been proven to improve the thermodynamic conditions for methane hydrates [22,23,24]. Additionally, they can lower the energy required for hydrate nucleation, resulting in more efficient and faster hydrate formation. These characteristics make THF and CP effective additives for enhancing the storage and transportation of methane using gas hydrates. 2,5-dihydrofuran and 2,3-dihydrofuran are heterocyclic compounds that share similarities with THF and CP, possessing comparable size, shape, and molecular weight [26]. Furthermore, similar to THF and CP, 2,5-dihydrofuran and 2,3-dihydrofuran are known to be capable of self-forming structure II hydrates [6, 7, 27, 28]. In this perspective, 2,5-dihydrofuran and 2,3-dihydrofuran can be considered as potential candidates, similar to THF and CP, for utilization in SNG applications. However, considering the lack of structural analysis studies on clathrate hydrates of 2,5-dihydrofuran and 2,3-dihydrofuran containing methane, this research aims to address this gap by conducting a comprehensive investigation.

This study aims to investigate the crystal structure of binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates. The characteristic behaviors of the 2,5-dihydrofuran and 2,3-dihydrofuran guest molecules in the hydrate cages were investigated via structural identification using high-resolution powder X-ray diffraction (HRPD) with Rietveld refinement.

Experimental Details

Distilled H2O (Samchun Pure Chemical, Pyeongtaek-si, South Korea), 2,5-dihydrofuran (98.0 mol% purity, Tokyo Chemical Industry), 2,3-dihydrofuran (98.0 mol% purity, Tokyo Chemical Industry), and CH4 gas (99.95 mol% purity, Korea Nano Gas) were used for synthesizing the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates.

To prepare the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates samples, a 5.56 mol% 2,5-dihydrofuran or 2,3-dihydrofuran solution was placed in a freezer (~ 193 K) under atmospheric pressure for one hour. And then, the frozen sample was ground to 100 μm under liquid nitrogen condition. Here, we used half of the resulting fine powder as (2,5-dihydrofuran + H2O) and (2,3-dihydrofuran + H2O) hydrate (without methane help gas) samples. And then, remaining half of the resulting fine powder was put into the pressure vessel, and then the reactor vessel was pressurized with CH4 up to 8.0 MPa. The reactor vessel was kept in a bath circulator (~ 269 K) for a week for the formation of the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates samples. The formed hydrate samples were recovered under liquid nitrogen condition, and then ground again.

For the structure identifications of as (2,5-dihydrofuran + H2O) and (2,3-dihydrofuran + H2O) hydrate, and of the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) hydrates, high-resolution powder X-ray diffraction (HRPD) patterns of our hydrate samples were acquired at 100 K using the 2D Supramolecular Crystallography beamline (with a synchrotron radiation of 0.90000 Å) at the Pohang Accelerator Laboratory (PAL) [29]. The detailed experimental procedures can be seen in our previous studies [30,31,32,33]. The obtained HRPD patterns were refined using the FullProf program and the FOX program for the Rietveld analyses with Direct Space method [34,35,36,37].

Results & Discussion

The crystal structure and guest inclusion behaviors of the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates were investigated via structural identification using high-resolution powder X-ray diffraction (HRPD) with Rietveld refinement (with Direct Space method) [34, 35]. The structural refinement of our HRPD patterns was performed using the FullProf program and the FOX program [36, 37]. The structural model of guest molecules (2,5-dihydrofuran, 2,3-dihydrofuran, and methane) was geometrically optimized using the CASTEP module with the GGA-BLYP model in the Materials Studio program (Fig. 1) [38]. 2,5-dihydrofuran and 2,3-dihydrofuran are similar to THF and CP, respectively, with comparable sizes, shapes, and molecular weights. In addition, 2,5-dihydrofuran and 2,3-dihydrofuran are known to be a structure II (sII) hydrate-forming agent without the help gas [6, 7, 27, 28]. Therefore, we also identified the crystal structure of (2,5-dihydrofuran + H2O) and (2,3-dihydrofuran + H2O) hydrates.

Fig. 1
figure 1

The optimized molecular structure of 2,5-dihydrofuran, 2,3-dihydrofuran, and methane and their molecular size (Gray, carbon; white, hydrogen; red, oxygen)

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In our study, the virtual atomic species, such as CH4, –CH2–, –CH–, and H2O, were used by using the sum of individual atomic scattering factors. Followings are the refined variables: zero shift, scale, peak shape, and thermal displacement parameters, atomic coordinates, lattice parameters, and site occupancies. The 2,5-dihydrofuran or 2,3-dihydrofuran molecule in the hydrate cages was regarded as the rigid body and positioned at the center of the large 51264 cages of sII hydrate. And then, the position of 2,5-dihydrofuran or 2,3-dihydrofuran molecule in the large 51264 cages of sII hydrate was estimated via the Direct Space method [35]. CH4 molecule can be captured in the small 512 and large 51264 cages of sII hydrate for the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates, and thus, the ratio of the CH4 molecules in the small 512 and large 51264 cages of sII hydrate was assumed to be 10.98 and 8.13 for the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates from the area ratio (AsII-S/AsII-L) from the 13C solid-state nuclear magnetic resonance (NMR) results (Figure S1).

In Fig. 2, the refined HRPD patterns of (2,5-dihydrofuran + H2O) hydrate (Fig. 2a) and (2,5-dihydrofuran + methane) hydrate (Fig. 2b) are shown and the refined patterns are good with reliability factors (background corrected Rwp = 10.5% and χ2 = 13.3 for Fig. 2a and Rwp = 13.4% and χ2 = 37.6 for Fig. 2b). In Tables 1 and 2, the atomic coordinates, thermal displace parameters, and site occupancies of the refined (2,5-dihydrofuran + H2O) hydrate (Fig. 2a) and (2,5-dihydrofuran + methane) hydrate (Fig. 2b) are listed. The refined results showed that the crystal structure of (2,5-dihydrofuran + H2O) hydrate (Fig. 2a) and (2,5-dihydrofuran + methane) hydrate (Fig. 2b) was identified as the cubic Fd-3m space group with lattice parameters of 17.13486(36) and 17.06365(59) Å. In addition, the hexagonal P63/mmc structure of the hexagonal ice could be also observed in the refined PXRD patterns (Fig. 2). The calculated weight fraction of (2,5-dihydrofuran + H2O) hydrate was approximately 87.34%, with the impurity hexagonal ice (Ih) for the remaining 12.66%. And the calculated weight fraction of the binary (2,5-dihydrofuran + methane) hydrate was 98.34%, with the impurity hexagonal ice (Ih) for the remaining 1.66%. Similarly, Fig. 3 shows the refined HRPD patterns (with reliability factors, background corrected Rwp = 11.0% and χ2 = 19.5 for Fig. 3a and Rwp = 13.5% and χ2 = 33.7 for Fig. 3b) of (2,3-dihydrofuran + H2O) hydrate and (2,3-dihydrofuran + methane) hydrate (Tables 3 and 4). The lattice parameters for (2,3-dihydrofuran + H2O) hydrate and (2,3-dihydrofuran + methane) hydrate were reported as 17.12526(51) and 17.04578(66) Å. The calculated weight fraction of (2,3-dihydrofuran + H2O) hydrate was approximately 63.16%, with the impurity hexagonal ice (Ih) for the remaining 36.84%. Similarly, the calculated weight fraction of the binary (2,3-dihydrofuran + methane) hydrate was 66.13%, with the impurity hexagonal ice (Ih), and structure I hydrate accounting for the remaining 31.87%, and 2.01%, respectively. One notable thing is the decrease of lattice parameters during the methane encapsulation in the hydrate cages for both 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrates. Kawamura et al. [39] reported that the lattice parameters of THF and 2,5-dihydrofuran clathrate hydrates decreased during the enclathration of hydrogen molecules in the empty 512 cages of sII hydrate. Similarly, the decrease of lattice parameters in our hydrate systems during the enclathration of methane molecules in the hydrate cages can be observed, and these phenomena may be related to the inter-molecule interaction between methane guest molecule and host water or 2,5-dihydrofuran (or 2,3-dihydrofuran) [39]. To clearly understand the host–guest and guest–guest interactions by adding the methane molecule in the hydrate system, further studies using ab initio modeling should be performed in the future.

Fig. 2
figure 2

Supramolecular crystallography patterns of a (2,5-dihydrofuran + H2O) hydrate measured at 100 K and the Rietveld refinement results (background corrected Rwp = 10.5% and χ2 = 13.3), and b (2,5-dihydrofuran + methane) hydrate measured at 100 K and the Rietveld refinement results (background corrected Rwp = 13.4% and χ2 = 37.6), Tick marks indicate the Bragg position for sII hydrate and the hexagonal ice phases

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Table 1 Atomic coordinates, isotropic temperature factors, and site occupancies for (2,5-Dihydrofuran + H2O) hydrate (Wa, virtual atomic species for the host framework; LG, virtual atomic species for 2,5-Dihydrofuran)
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Table 2 Atomic coordinates, isotropic temperature factors, and site occupancies for (2,5-Dihydrofuran + CH4) hydrate (Wa, virtual atomic species for the host framework; SG, virtual atomic species for methane; LG, virtual atomic species for 2,5-Dihydrofuran)
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Fig. 3
figure 3

Supramolecular crystallography patterns of a (2,3-dihydrofuran + H2O) hydrate measured at 100 K and the Rietveld refinement results (background corrected Rwp = 11.0% and χ2 = 19.5), and b (2,3-dihydrofuran + methane) hydrate measured at 100 K and the Rietveld refinement results (background corrected Rwp = 13.5% and χ2 = 33.7), Tick marks indicate the Bragg position for sII hydrate, the hexagonal ice, and sI hydrate phases

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Table 3 Atomic coordinates, isotropic temperature factors, and site occupancies for (2,3-Dihydrofuran + H2O) hydrate (Wa, virtual atomic species for the host framework; SG, virtual atomic species for methane; LG, virtual atomic species for 2,3-Dihydrofuran)
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Table 4 Atomic coordinates, isotropic temperature factors, and site occupancies for (2,3-Dihydrofuran + H2O) hydrate (Wa, virtual atomic species for the host framework; SG, virtual atomic species for methane; LG, virtual atomic species for 2,3-Dihydrofuran)
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In Figs. 4 and 5, 2,5-dihydrofuran, 2,3-dihydrofuran, and methane molecules with full symmetries in the large 51264 (for 2,5-dihydrofuran and 2,3-dihydrofuran) and small 512 (for methane molecules) cages of sII hydrates are given. During the Rietveld analysis of our HRPD patterns, the position of 2,5-dihydrofuran or 2,3-dihydrofuran molecule in the large 51264 cages of sII hydrate was estimated via the Direct Space method (using FOX program) [35, 37]. And then, the results showed that the 2,5-dihydrofuran or 2,3-dihydrofuran molecule in the case of (2,5-dihydrofuran + H2O) or (2,3-dihydrofuran + H2O) clathrate hydrates is almost positioned at the center of large 51264 cages of sII hydrate (Figs. 4a and 5a), but the 2,5-dihydrofuran or 2,3-dihydrofuran molecule in the case of the binary (2,5-dihydrofuran + CH4) and (2,3-dihydrofuran + CH4) clathrate hydrates is off-centered at the center of large 51264 cages of sII hydrate (Figs. 4b and 5b). Therefore, the distances between host and guest molecules in the large 51264 cages of sII hydrate for the binary (2,5-dihydrofuran + CH4) and (2,3-dihydrofuran + CH4) clathrate hydrates also changed (Figs. 6 and 7). Figure 6 shows that the shortest distances between host water and guest 2,5-dihydrofuran molecules decrease from 3.32 to 2.91 Å during the enclathration of methane molecule in the hydrate cages [40]. Similar trends are observed in Fig. 7 for 2,3-dihydrofuran hydrate systems (3.36 to 2.78 Å) during the enclathration of methane molecule. With the decrease of the shortest distances between host water and guest molecules in the large 51264 cages of sII hydrate, we may clearly conclude that there is the increase of guest–host interaction via possible hydrogen bonding interactions during the methane inclusion behaviors. Again, the enclathration of methane molecules in the hydrate cages may affect the inter-molecule interaction between host–guest and guest–guest molecules, and the unanswered nature of host-guest inclusion chemistry should be clearly understood in the future.

Fig. 4
figure 4

Crystal structure and guest positions of a (2,5-dihydrofuran + H2O) hydrate, and b (2,5-dihydrofuran + methane) hydrate obtained by Rietveld analysis. Distribution of guest molecules in small and large cages of (2,5-dihydrofuran + H2O) hydrate, and (2,5-dihydrofuran + methane) hydrate with full symmetry

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Fig. 5
figure 5

Crystal structure and guest positions of a (2,3-dihydrofuran + H2O) hydrate, and b (2,3-dihydrofuran + methane) hydrate obtained by Rietveld analysis. Distribution of guest molecules in small and large cages of (2,3-dihydrofuran + H2O) hydrate, and (2,3-dihydrofuran + methane) hydrate with full symmetry

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Fig. 6
figure 6

The 2,5-Dihydrofuran in in the large (51264) cage of a (2,5-dihydrofuran + H2O) hydrate, and b (2,5-dihydrofuran + methane) hydrate obtained by Rietveld analysis (gray, oxygen in water host molecule; blue, oxygen in guest molecule; red, carbon in guest molecule; hydrogen atoms in water host molecule and guest molecule are omitted)

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Fig. 7
figure 7

The 2,3-Dihydrofuran in in the large (51264) cage of a (2,3-dihydrofuran + H2O) hydrate, and b (2,3-dihydrofuran + methane) hydrate obtained by Rietveld analysis (gray, oxygen in water host molecule; blue, oxygen in guest molecule; red, carbon in guest molecule; hydrogen atoms in water host molecule and guest molecule are omitted)

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In present studies, we identified the crystal structure of 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrate systems with/without methane help gas. The characteristic behaviors of the 2,5-dihydrofuran and 2,3-dihydrofuran guest molecules with/without methane help gas in the hydrate cages were demonstrated via Rietveld analysis with the Direct Space method. The results of this study may provide the useful information on the unique nature of host–guest inclusion compounds.

Conclusions

In summary, we identified the crystal structure of 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrate systems with/without methane help gas. The crystal structure of 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrate systems with/without methane help gas represents the structure II clathrate hydrate with the cubic Fd-3m space group. During the methane inclusion behavior, we observed the decrease of lattice parameters for both 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrates. The position of 2,5-dihydrofuran or 2,3-dihydrofuran molecule is almost at the center of large 51264 cages of (2,5-dihydrofuran + H2O) or (2,3-dihydrofuran + H2O) clathrate hydrates, but is off-centered at the center of large 51264 cages of the binary (2,5-dihydrofuran + CH4) or (2,3-dihydrofuran + CH4) clathrate hydrates. And then, there is an increase of guest–host interaction via possible hydrogen bonding interactions during the methane inclusion behaviors. These results provide useful insights into the complex nature of host–guest inclusion chemistry.