Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

Abstract

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high-resolution neutron powder diffraction. Three dihydrogen molecules (H(2), D(2), and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen-doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut-like motion of the HD guest in the center of anisotropic sII-S (sII-S=small cages of structure II hydrates). However, we failed to observe a mass-dependent relationship when comparing D(2) with HD. The use of asymmetric guest molecules can significantly contribute to tuning the cage dimension and thus can improve the stable inclusion of small gaseous molecules in confined cages.

Effects of Large Guest Molecular Structure on Thermal Expansion Behaviors in Binary (C4H8O + CH4) Clathrate Hydrates

“…Considering the RDFs in Figure , dispersed molecular motion and thermal expansion of a large guest make the butyraldehyde closer to the host cage, and this can lead to a larger attractive or repulsive force. This supposition corresponds to a previous study that the molecular motion of the guest molecule in the hydrate cage affects the thermal expansivity . Also, Tse et al reported that the guest–host interaction enhances the anharmonic terms of the hydrate crystal potential and causes larger thermal expansivity .…”

“…This supposition corresponds to a previous study that the molecular motion of the guest molecule in the hydrate cage affects the thermal expansivity. 56 Also, Tse et al reported that the guest−host interaction enhances the anharmonic terms of the hydrate crystal potential and causes larger thermal expansivity. 57 Accordingly, through the simulation results in Figures 7 and 8, it can be speculated that the molecular thermal expansivity and motions varied with the structure of the large guest molecules, resulting in differences in guest−host interactions, hydrate crystal potential, and thermal expansivity of the lattice.…”

Thermodynamic and Spectroscopic Identification of Methane Inclusion in the Binary Heterocyclic Compound Hydrates

“…Both molecular mass and motion of guest molecules in the hydrate cavities could determine the cavity size and the corresponding structure of hydrates. However, the molecular size of a hydrate former has been recognized as a key factor in determining the structure of clathrate hydrates. ,, A molecular size above 7.5 Å is too large to be captured into the small cages of structure II (sII) hydrate. ,− The molecular size of pyridine and pyrrole was determined through the calculation of optimum molecular structures by using Gaussian 03 . The B3LYP method with the 6-311 ++ G (d,p) basis set was used for this calculation.…”

“…However, the molecular size of a hydrate former has been recognized as a key factor in determining the structure of clathrate hydrates. 1,21,22 A molecular size above 7.5 Å is too large to be captured into the small cages of structure II (sII) hydrate. 1,23−25 The molecular size of pyridine and pyrrole was determined through the calculation of optimum molecular structures by using Gaussian 03.…”