Thermal Expansivity of Tetrahydrofuran Clathrate Hydrate with Diatomic Guest Molecules

Abstract

The guest dynamics and thermal behavior occurring in the cages of clathrate hydrates appear to be too complex to be clearly understood through various structural and spectroscopic approaches, even for the well-known structures of sI, sII, and sH. Neutron diffraction studies have recently been carried out to clarify the special role of guests in expanding the host water lattices and have contributed to revealing the influence factors on thermal expansivity. Through this letter we attempt to address three noteworthy features occurring in guest inclusion: (1) the effect of guest dimension on host water lattice expansion; (2) the effect of thermal history on host water lattice expansion; and (3) the effect of coherent/incoherent scattering cross sections on guest thermal patterns. The diatomic guests of H 2, D 2, N 2, and O 2 have been selected for study, and their size and mass dependence on the degree of lattice expansion have been examined, and four sII clathrate hydrates with tetrahydrofuran (THF) have been synthesized in order to determine their neutron powder diffraction patterns. After thermal cycling, the THF + H 2 clathrate hydrate is observed to exhibit an irreversible plastic deformation-like pattern, implying that the expanded lattices fail to recover the original state by contraction. The host-water cage dimension after degassing the guest molecules remains as it was expanded, and thus host-guest as well as guest-guest interactions will be altered if guest uptake reoccurs.

Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

“…Here, we note that the lattice dimension difference between (THF+H 2 )* and (THF+D 2 )* results from the internal pressure difference induced by the mass-dependent thermal energy ratio of H 2 /D 2 = 1:2. [3] Unexpectedly, we failed to see this type of mass-dependent relationship for the two different clathrate hydrates of (THF+D 2 )* and (THF+HD)*. Although HD has a lower molecular weight than D 2 , the unit-cell parameter of the (THF+HD)* hydrate (a = 17.07824 , T = 30 K) is larger than that of the (THF+D 2 )* hydrate.…”

“…We directly used the previous procedure to obtain the thermal expansion coeffi-Abstract in Korean: cients of (THF+H 2 , D 2 and HD)*, which were then plotted as a function of temperature ( Figure S4 and Table S5 in the Supporting Information). [3,15] At 30 K, we observed a smaller lattice parameter of the (THF + H 2 )* hydrate (a = 17.06542 ) than that of (THF+D 2 )* (a = 17.07489 ). This discrepancy continued to high temperature (T = 273 K), as shown in Figure 2 a.…”

“…[13] In addition, in a recent neutron powder diffraction (NPD) study, the guest mass was evaluated as a second option for determining the unit cell parameter of sII clathrate hydrates with diatomic guest molecules. [3] We particularly noted that the direct use of hydrogen gas in the NPD study was successfully performed, showing stable background signals without any deuteration process. [4] These unique guest dynamics further stimulated us to explore the structural characteristics of deuterium hydride (HD) in the confined cages.…”

“…We also note that inherent molecular details of encaged guests such as ionic/nonionic and hydrophilic/hydrophobic guests greatly influence the cage dynamics. [2][3][4][5][6] Recently, hydrogen has received a great deal of attention in the hydrate community owing to its high potential for use in energy-related and many other issues. [7,8] Numerous studies concerning hydrogen storage using clathrate hydrates have been reported with much efforts aimed at increasing storage capacity, introducing unusual tuning effects by changes in concentration and pressure.…”

“…[2,12] In particular, we also noted a thermal behavior during atomic or molecular guest inclusion in which, after thermal cycling, the clathrate hydrate structure was observed to exhibit an irreversible plastic deformation-like pattern, thus implying that the expanded lattices failed to recover to their original state by contraction. [3] The host-water cage dimension after the degassing of guest molecules remains as it was in its expanded state. Thus, host-guest as well as guest-guest interactions are altered if guest uptake reoccurs.…”