A Monte Carlo Simulation
Study of Methane Clathrate
Hydrates Confined in Slit-Shaped Pores
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Abstract
Monte Carlo simulations are used to study the structure,
stability,
and dissociation mechanisms of methane hydrate crystals inside carbon-like
slit-shaped pores. The simulation conditions used mimic experimental
studies of the dissociation of methane and propane hydrates in mesoporous
silica gels (Handa, Y. P.; Stupin, D. <i>J. Phys. Chem. </i><b>1992</b>, <i>96</i>, 8599). Simulations are performed
under conditions of fixed methane pressure and fixed water loading,
with the temperature increased in steps, with long equilibrations
at each temperature. The initial structures of the confined hydrates
are taken to be bulk-like, and pore widths chosen to accommodate integer
or half-integer numbers of hydrate unit cells. Density profiles and
orientational order parameter profiles are obtained and used to understand
the structural changes associated with hydrate dissociation. Three
different common water models, SPC/E, TIP4P, and TIP4P/2005, are used
and the results compared. For water modeled using either the TIP4P
or TIP4P/2005 potentials, dissociation temperatures are depressed
proportionally to the inverse pore width, as predicted by the macroscopic
Gibbs–Thomson equation. This behavior is observed for pores
small enough that only half-cages of the clathrate structure are present.
Experimental work has verified Gibbs–Thomson behavior for pores
as small as 2 nm (Seshadri, K.; Wilder, J. W.; Smith, D. H. <i>J. Phys. Chem. B</i> <b>2001</b>, <i>105</i>, 2627); micropores of the size studied here have not yet been studied
by experiment. Interestingly, the dissociation of hydrates modeled
using the SPC/E water potential does not display the predicted pore-size
dependence, and the dissociation mechanisms in this model seem to
be quite different than those in the TIP4P-type models. In the SPC/E
hydrates, with increasing temperature, cage dissocation occurs before
methane desorption. In TIP4P-type hydrates, these processes occur
either at the same temperature (to within the resolution of this study)
or with dissociation occurring at higher temperatures than desorption.
These simulations show that a variety of interesting clathrate structures
and phase behaviors may be accessed in suitably designed microporous
materials, with potentially useful applications in gas storage or
separations