Ageing and Langmuir Behavior of the Cage Occupancy in the Nitrogen Gas Hydrate

Abstract

Abstract:Clathrate hydrates are ice-like systems in which nanometric water cages encapsulate guest molecules. Functionalizing clathrate hydrates is an important issue, accomplished by playing with their chemical composition and their cage structure. In this issue, the cage occupancy and its kinetics constitute key information for future developments. In many aspects, nitrogen gas hydrate represents an interesting system not only for its applied relevance (e.g., gas separation and methane/carbon dioxide exchange), but also for its fundamental interest (e.g., structural metastability and kinetics). Thanks to the complementarity of neutron diffraction and Raman scattering, the vibrational signatures of the so-called SI and SII clathrate structures of the nitrogen hydrates are reviewed. Moreover, the investigation of the ageing of the SII structure is reported together with its interpretation in the frame of the Langmuir behavior of the cage filling at low temperature. The cage filling is monitored with the help of a time-dependent analysis of the Raman scattering signals (over several months). The SII large cage filling decreases with a kinetic rate of 5.9 ± 3.3 × 10 −3 h −1 at 77 K and atmospheric pressure, so that equilibrium is reached after ca. eight weeks. Isotherm measurements of the guest Raman signatures lead to revealing a Langmuir constant higher in the small cage than in the large cage at 150 K. Such a behavior might thus be correlated with the nitrogen depletion with time, observed in the large cage of the SII nitrogen hydrate.

Nitrogen Hydrate Cage Occupancy and Bulk Modulus Inferred from Density Functional Theory-Derived Cell Parameters

“…14,15 Experimentally, it has been shown that the LCs in both hydrates have the ability to catch and release guest molecules as the pressure and the temperature vary. 13,15,20 Together with previous DFT calculations performed on carbon monoxide hydrate, 14,39 the present investigation clearly underlines that the cage occupancy is the factor driving the stability, i.e. leading to the lower potential energy of the formed hydrate structure.…”

“…As reported from time-dependent neutron diffraction analysis, 13 nitrogen hydrate forms in the sI clathrate metastable structure and transforms into the thermodynamically stable sII clathrate structure for which the cage occupancy plays a role. 20 Such structural metastability involves different potential energies for the sI and sII structures. The nitrogen hydrate stability has been analyzed in terms of DFT-calculated potential energies by following the methodology detailed in the work of Petuya et al 39 The potential energy considered for the present analysis corresponds to intermolecular energies whose reference states are defined on the basis of the intramolecular energies of the isolated molecular species (water and nitrogen).…”

“…Nitrogen hydrate initially forms in the sI structure that transforms into the sII structure after few days. 13,20 Carbon monoxide hydrate exhibits similar structural metastability as nitrogen hydrate, although its transformation from sI to sII is much longer and takes several weeks. 14,15 Experimentally, it has been shown that the LCs in both hydrates have the ability to catch and release guest molecules as the pressure and the temperature vary.…”

“…For instance, empty sI SCs correspond to θ SC = 0 (all SC empty) and one guest molecule per sI SC corresponds to θ SC = 1 (all SCs filled). Experimental studies with various techniques have been conducted to investigate the occupancy θ SC and θ LC of small and LCs, respectively, in nitrogen clathrate hydrates. ,,,,, These studies revealed that N 2 molecules were localized in both types of cages, regardless of the structure formed. Notably, neutron diffraction studies showed that a sII nitrogen hydrate formed at 150 bar and 258 K for 20 days had θ SC = 0.87 and θ LC = 1.2 (meaning that 20% of the sII LCs are occupied by two nitrogen molecules) and that a sI nitrogen hydrate formed at 1093 bar and 268 K for 11 days had θ SC = 0.98 and θ LC = 1.12 .…”

Molecular Selectivity of CO–N2 Mixed Hydrates: Raman Spectroscopy and GCMC Studies

“…Worthwhile to mention is, that pure clathrates of CO and N 2 are first formed in the sI-structure and they transform, in time, to the thermodynamically stable phase sII. [22][23][24][25] In the case of the mixed CO-N 2 hydrates 26 , this structural change promotes the CO molecular selectivity 17 . In this paper, this property is documented for the first time by combining Grand Canonical Monte Carlo (GCMC) simulations and Raman Scattering experiments.…”