A Monte Carlo Simulation Study of Methane Clathrate Hydrates Confined in Slit-Shaped Pores

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. J. Phys. Chem. 1992, 96, 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. J. Phys. Chem. B 2001, 105, 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.

Constant pressure Gibbs ensemble Monte Carlo simulations for the prediction of structure I gas hydrate occupancy

“…However, molecular simulations, both Monte Carlo and molecular dynamics simulations, are also a valuable tool for the study of gas hydrate properties. Gas hydrate structure can be determined by crystallographic experiments, therefore computer simulations have been carried out to study hydrate formation/dissociation, gas adsorption into the hydrate phase, and to test the assumptions made in the development of vdW-P theory (for example : Tester et al, 1972;Sparks and Tester, 1992;Tanaka, 1998;Wierzchowski and Monson, 2007;Sizov and Piotrovskaya, 2007;Papadimitriou et al, 2008;Chakraborty and Gelb, 2012;Glavatskiy et al, 2012;Ravipati and Sudeep, 2013;Lasich et al, 2014). A distinct advantage of studying gas hydrate systems using molecular simulation is that it allows the study of hypothetical gas hydrate states (such as a zero occupancy hydrate) or systems under conditions that would be difficult and/or expensive to study experimentally.…”

Divergent Spatiotemporal Interaction of Angiotensin Receptor Blocking Drugs with Angiotensin Type 1 Receptor

“…Using the Desmond system builder, a 10 Å buffered orthorhombic system with periodic boundary conditions was constructed using a POPC lipid membrane 38–41 and an SPC explicit water solvent. 34 The overall charge was neutralized by 0.15 mol/L NaCl.…”

Phase behavior and kinetics properties of gas hydrates in confinement and its application

“…Additionally, Nath et al showed using Monte Carlo simulations of methane hydrates in confinement of various pore sizes (using TIP4P and TIP4P/2005 models), a depression in hydrate dissociation temperature of up to around 28 K for 5 nm pore diameter. 24…”

“…It was observed that there is a shift in gas hydrates dissociation temperature when the pore size is between 3 and 100 nm. Additionally, Nath et al showed using Monte Carlo simulations of methane hydrates in confinement of various pore sizes (using TIP4P and TIP4P/2005 models), a depression in hydrate dissociation temperature of up to around 28 K for 5 nm pore diameter 24 …”