Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

Abstract

Classical equilibrium molecular dynamics simulations have been performed to investigate dynamical properties of cage radial breathing modes and intra- and inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen–tetrahydrofuran sII hydrates at 0.05 kbar and up to 250 K. For the mixed H2–THF system in which there is single H2 occupation of the small cage (labelled “1SC 1LC”), we find that no H2 migration occurs, and this is also the case for pure H2 hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed “1SC 4LC”). However, for the more densely filled H2–THF and pure-H2 systems, in which there is double H2 occupation in the small cage (dubbed “2SC 1LC” and “2SC 4LC,” respectively), there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K, with an approximate Arrhenius temperature-dependence for the migration rate from 200 to 250 K. It was found that these “cage hopping” events are facilitated by temporary openings of pentagonal small-cage faces with the relaxation and reformation of key stabilising hydrogen bonds during and following passage. The cages remain essentially intact up to 250 K, save for transient hydrogen bond weakening and reformation during and after inter-cage hydrogen diffusion events in the 200–250 K range. The “breathing modes,” or underlying frequencies governing the variation in the cavities’ radii, exhibit a certain overlap with THF rattling motion in the case of large cavities, while there is some overlap of small cages’ radial breathing modes with lattice acoustic modes.

Diffusive hydrogen inter-cage migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

“…Inter-cage migration has been observed by Gorman et al [21], as shown by Figure 2 of that article [21], as well as by Frankcombe and Kroes [20]. In ref.…”

“…In the latter case there was no migration detected for the hydrogen molecules (See Figure 5). The single hydrogen molecule in the small cages in both systems 1S1L and 1S4L are largely non-percolating (as also observed in [21]). More vigorous vibration of the large cages due to quadruple occupation in the case of 1S4L, particularly at higher temperatures, coupled with single-occupation of the small cages allows for limited hopping to occur between small and large cages.…”

“…We wish to study the influence of temperature and the hydrogen occupation of the small cavities (i.e., governed by the ratio of singly-to doubly-occupied cavities -varying this ratio for different overall smallcavity occupations). Rovetto et al [15] found that triple occupancy is unlikely, and we only observed transient instances of this in previous work [21]. Therefore, we have carried out equilibrium MD at 50 bar, of interest to industrial storage, and from 200 up to 260 K for different small-cage hydrogen occupancies, including both pure and mixed THF-H 2 clathrates, and we have applied the anomalous-Fickian diffusion-modelling approach of McDermott et al [27] to further characterise and quantify the inter-cage hopping diffusion rates observed in our previous study [21], and ascertain thereon the effect of small cage H 2 occupancy.…”

“…However, occupation of up to six hydrogen molecules in a large cage was observed by Frankcombe and Kroes, with lifetimes of up to hundreds of picoseconds [20]. In addition, Gorman et al [21] have studied hydrogen migration in clathrates, with a similar lack of observations of hopping in systems with singly-occupied small cavities. Nevertheless, hydrogen migration was detected in doubly occupied systems, with H 2 motion transverse to the pentagonal face into the neighbouring cages.…”

“…In ref. 21, the hydrogen molecules were often observed to move to the pentagonal face of the small cage and then moved back to the geometric centre of the cage; this intra-cage, non-percolating movement occurred rapidly. Occasionally, the hydrogen molecule that moved to the surface of the small cage may weaken the stabilising hydrogen bonds, which may result in the temporary opening of the pentagonal face to allow the hydrogen molecule to 'escape' from the cage.…”