Guest–Guest Interactions and Co-Occupation by Distinct Guests in the Metastable State of Clathrate Hydrates

Abstract

The current knowledge of guest−guest interactions and co-occupation in clathrate hydrates is exclusive for the same guests (H 2 or N 2 ) at moderate pressure. Here, we introduce the unusual co-occupation of distinct guests in the metastable state of hydrates. With controlled hydrate fraction, particle size, and intensification of the sintering of SF 6 hydrate particles formed from water and SF 6 gas as a help gas, we observed an abnormal but unique synchronous behavior in Raman intensities of two guest molecules (SF 6 and N 2 /H 2 ) in hydrates consistently and repeatedly; over time, the scattering intensity for the guests (i) increases, (ii) decreases, and (iii) finally reaches the stable level. Without a concentration change of SF 6 , this abnormal behavior must arise from the possible changes in the scattering cross section of the molecules, suggesting that N 2 /H 2 strongly interacts with SF 6 in the large cages, resulting in a possible co-occupation during the metastable transition. These observations on the metastability of gas hydrate attest the importance of the sintering effect as a barrier to prevent fast gas diffusion for reaching equilibrium, which could have significant implication in increasing overall gas storage in clathrate hydrates.

Characterization of Hydrate Formation and Flow Influenced by Hydrophilic–Hydrophobic Components within a Fully Visual Rocking Cell

“…It should be noted that during the process of cooling, accompanied by the gas contraction and dissolution in the liquid within the rocking cell, the system pressure showed the first visible decrease. In the second period (B–C), both temperature and pressure remained stable as the system in metastability, , where hydrate had not yet formed and there was no gas consumption. In the third stage (C–F), a sharp drop in the pressure curve and a significant increase in the water conversion curve were observed from point C, indicating that point C was the onset of hydrate formation.…”

A comprehensive review on molecular dynamics simulation studies of phenomena and characteristics associated with clathrate hydrates

“…To date, a number of materials/ techniques have been suggested, and a hydrate-based methodology would be the proper option. Since pure H2 hydrate requires uneconomic conditions, the presence of additives as promoters such as TBAB [127], THF [128], or SF6 [129] have been suggested. Given that identifying the key controllers of the kinetics and thermodynamics of gas hydrates in contact with promoters for the suggested hydrate-based applications is particularly critical.…”

“…Gas hydrates can also be useful for gas storage aims, however, the common challenge associated with this is to balance the trade-off between thermodynamic efficiency and H2 storage capacity. Raman spectroscopic measurements revealed that if sII SF6 hydrate is exposed to H2 molecules in the vapor phase, as time proceeds, the H2 concentration in hydrate will hit a peak and then reduce before reaching a stable value [129]. Although this phenomenon cannot be fully explained through spectroscopy experiments, its dynamic process was recently explored through MD simulations in which every large cavity was initially co-occupied by both SF6 and H2 molecules.…”

A comprehensive review of hydrogen production and storage: A focus on the role of nanomaterials

“…Within 2 h, a H2 uptake of up to 26.51 v/v (gas/hydrate) was attained, corresponding to a 30.2% increase over the maximum H2 uptake obtained using the conventional THF promoter. Besides the above-mentioned promoters, other suggested promoters explored in the literature include tertiary alcohols [234], argon [235] , nitrogen [236], alkyl amines [237], methane [238], and SF6 [239]. A summary of important promoter molecules used in the past 20 years and their H2 storage properties is presented in Gupta et al [179].…”