Rietveld Analysis of Binary (2,5-Dihydrofuran + Methane) and (2,3-Dihydrofuran + Methane) Clathrate Hydrates

Herein, we examined the crystal structure of 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrate systems in the presence of methane as help gas. The crystal structure of these systems demonstrates the structure II (sII) clathrate hydrate with the cubic Fd-3m space group. Throughout the inclusion of methane, we observed a decrease in lattice parameters for both 2,5-dihydrofuran and 2,3-dihydrofuran clathrate hydrates. In the (2,5-dihydrofuran + H 2 O) or (2,3-dihydrofuran + H 2 O) clathrate hydrates, the 2,5-dihydrofuran or 2,3-dihydrofuran molecule is located at the center of the large cages of sII hydrate. However, in the (2,5-dihydrofuran + CH 4 ) or (2,3-dihydrofuran + CH 4 ) binary clathrate hydrates, the 2,5-dihydrofuran or 2,3-dihydrofuran molecule is positioned off-center in the large cages of sII hydrate. Finally, we con�rmed the possibility increase of host-guest interaction via possible host–guest hydrogen bonding due to the decrease of the shortest distance between host and guest molecules.

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 e cient 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 (2,3dihydrofuran + methane) clathrate hydrates.The characteristic behaviors of the 2,5-dihydrofuran and 2,3dihydrofuran guest molecules in the hydrate cages were investigated via structural identi cation using high-resolution powder X-ray diffraction (HRPD) with Rietveld re nement.
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 ne powder as (2,5-dihydrofuran + H 2 O) and (2,3dihydrofuran + H 2 O) hydrate (without methane help gas) samples.And then, half of the resulting ne powder was put into the pressure vessel, and then the reactor vessel was pressurized with CH 4 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.
In our study, the virtual atomic species, such as CH 4 , -CH 2 -, -CH-, and H 2 O, were used by using the sum of individual atomic scattering factors.Followings are the re ned 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 5 12 6 4 cages of sII hydrate.And then, the position of 2,5-dihydrofuran or 2,3-dihydrofuran molecule in the large 5 12 6 4 cages of sII hydrate was estimated via the Direct Space method [35].CH 4 molecule can be captured in the small 5 12 and large 5 12 6 4 cages of sII hydrate for the binary (2,5-dihydrofuran + methane) and (2,3-dihydrofuran + methane) clathrate hydrates, and thus; the ratio of the CH 4 molecules in the small 5 12 and large 5 12 6 4 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 (A sII-S /A sII-L ) from the 13 C solid-state nuclear magnetic resonance (NMR) results (Figure S1).
In present studies, we identi ed 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.

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Figure 5 Crystal
Figure 5
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 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 5 12 6 4 cages of (2,5dihydrofuran + H 2 O) or (2,3-dihydrofuran + H 2 O) clathrate hydrates, but is off-centered at the center of system in KBSI Seoul Western Center.Powder X-Ray diffraction (PXRD) patterns are collected from the beamline (2D) at Pohang Accelerator Laboratory.