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Publisher DOI: 10.1021/ja802014m
Title: Path sampling calculation of methane diffusivity in natural gas hydrates from a water-vacancy assisted mechanism
Language: English
Authors: Peters, Baron 
Zimmermann, Nils E. R. 
Beckham, Gregg T. 
Tester, Jefferson W. 
Trout, Bernhardt L. 
Keywords: Chemical engineering; Carbon sequestration; Hydrate; Methane; Transport; Molecular dynamics; Rare event simulations
Issue Date: 3-Dec-2008
Publisher: American Chemical Society
Source: J. Am. Chem. Soc. 130 (51): 17342–17350 (2008)
Abstract (english): 
Increased interest in natural gas hydrate formation and decomposition, coupled with experimental difficulties in diffusion measurements, makes estimating transport properties in hydrates an important technological challenge. This research uses an equilibrium path sampling method for free energy calculations [R. Radhakrishnan and T. Schlick, J. Chem. Phys. 121 2436 (2004)] with reactive flux and kinetic Monte Carlo simulations to estimate the methane diffusivity within a structure I gas hydrate crystal. The calculations support a water-vacancy assisted diffusion mechanism where methane hops from an occupied “donor” cage to an adjacent “acceptor” cage. For pathways between cages that are separated by five-membered water rings, the free energy landscape has a high barrier with a shallow well at the top. For pathways between cages that are separated by six-membered water rings, the free energy calculations show a lower barrier with no stable intermediate. Reactive flux simulations confirm that many reactive trajectories become trapped in the shallow intermediate at the top of the barrier leading to a small transmission coefficient for these paths. Stable intermediate configurations are identified as doubly occupied off-pathway cages and methane occupying the position of a water vacancy. Rate constants are computed and used to simulate self-diffusion with a kinetic Monte Carlo algorithm. Self-diffusion rates were much slower than the Einstein estimate because of lattice connectivity and methane’s preference for large cages over small cages. Specifically, the fastest pathways for methane hopping are arranged in parallel (non-intersecting) channels, so methane must hop via a slow pathway to escape the channel. From a computational perspective, this paper demonstrates that equilibrium path sampling can compute free energies for a broader class of coordinates than umbrella sampling with molecular dynamics. From a technological perspective, this paper provides one estimate for an important transport property that has been difficult to measure. In a hydrate I crystal at 250 K with nearly all cages occupied by methane, we estimate D ≈ 7·10-15 X m2/s where X is the fraction of unoccupied cages.
DOI: 10.15480/882.2822
ISSN: 1520-5126
Journal: Journal of the American Chemical Society 
Document Type: Article
License: In Copyright In Copyright
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