Abstract
Tetrahydrofuran (THF) is well known as a former and a promoter of clathrate hydrates, but the molecular mechanism for the formation of these compounds is not yet well understood. We performed ab initio calculations and ab initio molecular dynamics simulations to investigate the formation, structure, and stability of THF·(H2O)n=1–10 and its significance to the formation of the THF hydrate. Weak hydrogen bonds were found between THF and water molecules, and THF could promote water molecules from the planar pentagonal or hexagonal ring. As a promoter, THF could increase the binding ability of the CH4, CO2, or H2 molecule onto a water face, but could also enhance the adsorption of other THF molecules, causing an enrichment effect.
Highlights
Clathrate hydrates are ice-like crystalline compounds in which gas molecules are encaged in a host framework of water molecules, and they are widely found in permafrost and ocean floor sediments [1,2,3].Up to now, three common types of methane hydrate structures have been identified [4,5]: The cubic structure I, the cubic structure II, and the hexagonal structure H
The results showed that the hydrogen bonds between THF and water were relatively weak, with a maximum number of two water molecules hydrogen bonded with THF, but THF could facilitate the rearrangement of water molecules to form a pentagonal or hexagonal planar ring, which was responsible for the formation of a clathrate cage
This suggested that a quasiplanar cyclic structure of the water molecules would be energetically favorable, agreeing well with the theoretical results of Shields et al [37], and this structure would considerably occur during the formation of the clathrate cage
Summary
Clathrate hydrates are ice-like crystalline compounds in which gas molecules are encaged in a host framework of water molecules, and they are widely found in permafrost and ocean floor sediments [1,2,3]. Tetrahydrofuran (THF) molecules can form the sII hydrate at about room temperature and pressure [13], which can serve as a proxy for developing hydrate technology. Because of their large molecular size, THF molecules only occupy the large 512 64 cages, leaving the small 512 cages vacant. Alavi and coworkers [30] have studied the hydrogen bonding of THF in binary sII hydrates via molecular dynamics simulations, and suggested that the number and nature of the second guests can affect the probability of hydrogen bonding of THF. THF could significantly enhance the attraction of water faces to a second-guest component, which is helpful in understanding the promotion effects of THF on clathrate hydrates of various gas molecules
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