jp0c03938_si_001.pdf (1.24 MB)
Effect of Small Cage Guests on Dissociation Properties of Tetrahydrofuran Hydrates
journal contribution
posted on 2020-08-06, 22:00 authored by Kuang-Yu Chang, Che-Kang Chu, Lee-Shin Chu, Yen-An Chen, Shiang-Tai Lin, Yan-Ping Chen, Li-Jen ChenIt is well understood
that tetrahydrofuran (THF) molecules are
able to stabilize the large cages (51264) of
structure II to form the THF hydrate with empty small cages even at
atmospheric pressure. This leaves the small cages to store gas molecules
at relatively lower pressures and higher temperatures. The dissociation
enthalpy and temperature strongly depend on the size of gas molecules
enclathrated in the small cages of structure II THF hydrate. A high-pressure
microdifferential scanning calorimeter was applied to measure the
dissociation enthalpies and temperatures of THF hydrates pressurized
by helium and methane under a constant pressure ranging from 0.10
to 35.00 MPa and a wide THF concentration ranging from 0.25 to 8.11
mol %. The dissociation temperature of binary He + THF and methane
+ THF hydrates increases along with an increase in the THF concentration
in the liquid phase at a fixed pressure (e.g., 30 MPa), reaching a
maximum value of 280.8 and 312.8 K, respectively, at stoichiometric
concentration (5.56 mol % THF), and then remains nearly constant for
even higher THF concentrations (>5.56 mol %). The effect of gas
occupancy
in the small cages on the dissociation enthalpy of He + THF and methane
+ THF mixed hydrates was further examined by using molecular dynamics
(MD) simulations. The dissociation enthalpy of the He–THF mixed
hydrates is independent of pressure with an average of 5.68 kJ/mol
H2O over the pressure ranging from 0.10 to 30.0 MPa, consistent
with the MD results of the He–THF mixed hydrates with low single
occupancy (<23%) of helium molecules in the small cages. Consequently,
the heat of adsorption of helium molecules in the small cages of the
He–THF mixed hydrates is rather too weak to be identified.
On the other hand, the dissociation enthalpy of the methane–THF
hydrates increases from 9.11 to 10.01 kJ/mol H2O along
with an increase in methane pressure over the pressure ranging from
5.0 to 30.0 MPa, consistent with the MD results of the methane–THF
mixed hydrates with full occupancy of methane molecules in the small
cages. These findings provide important information for the design
of a potential medium of gas storage and transportation.