Modifying the Flexibility of Water Cages by Co-Including Acidic Species within Clathrate Hydrate

Abstract

Clathrate hydrates are crystalline materials made of water molecules forming host cages within which guest molecules are located. The hydrogen bond network ensuring the stability of the host substructure includes ionic defects, having an impact on the physicochemical properties of the systems. In this paper, a new way of introducing these ionic defects is proposed. Type II clathrate hydrates mixing tetrahydrofuran (THF) and perchloric acid guest molecules are synthesized and investigated by means of calorimetric, X-ray diffraction, and Raman scattering measurements together with a computational structure relaxation in the density functional theory approximation. The formation of the mixed clathrate hydrate with perchlorate anion included in the large cage of the cationic host-substructure of the THF type II clathrate hydrate requires the cooling of a (1-α) THF·αHClO4·17H2O solution with α less than 0.125. Above this inherent limitation, a multiphasic regime is observed in the formation of clathrate hydrate (mixture of type I and type II). The substitution of a THF molecule per perchlorate anion allows the modification of the melting of the type II clathrate hydrate, by preserving the clathrate structure. Shrinkage of the type II unit cell is measured together with a softening of the host lattice mode. In the harmonic approximation, the observation of both phenomena is counterintuitive and outline existing competition between anharmonicity of the cage energy landscape and ionic host–guest interaction. This study reveals the key role played by acidic defects existing in the host substructure on the physicochemical properties of clathrate hydrate.

Modeling the THF clathrate hydrate dynamics by combining molecular dynamics and quasi-elastic neutron scattering

“…This disorder of the H-bond network in the water substructure has been revealed by means of dielectric spectroscopy [34][35][36], of 1 H NMR [37] and of solid state 2 H NMR [38][39][40][41][42]. The timescale of the water dynamics ranges from the microsecond to the millisecond above 193 K. The impact of acidic additives -known to increase host proton mobility in the host substructure [43][44][45] -has been investigated on its thermodynamical, vibrational and structural properties [46]. Finally, the THF ⇑ Corresponding author.…”

Promoting the Insertion of Molecular Hydrogen in Tetrahydrofuran Hydrate With the Help of Acidic Additives

“…In the present investigation, two hydrates have been considered: the THF hydrate (formed with a THF·17H 2 O solution) and the mixed THF-HClO 4 hydrate (formed with a 0.875THF-0.125HClO 4 ·17H 2 O solution). In these sII hydrates, the SCs are empty to welcome H 2 molecules, the THF molecules occupy the LC and the acidic additive HClO 4 is inserted within the LC by replacing one THF per unit cell in average (Desmedt et al, 2015 ). The insertion of molecular hydrogen within these two preformed hydrates has been probed thanks to real-time Raman imaging.…”

“…They were transferred under inert atmosphere in the lab-made high-pressure optical cell used for the Raman spectroscopic analysis. According to a procedure previously published (Desmedt et al, 2015 ), the hydrates have been formed under stirring conditions by cooling the sample temperature to 270 K and maintaining this temperature for 24 h with the help of a modified cryogenic stage (Linkam Scientific Instruments Ltd., UK). Once the hydrate is formed, hydrogen gas (99.9999% Air Liquide) was then applied at constant pressure (200 bar) with a PM High Pressure pump (Top Industrie, Vaux-le-Penil, France) which contains 100 cm 3 of gas.…”

“…Recently, it has been shown that water molecules relax on a nanosecond timescale in strong acid clathrate hydrates (Desmedt et al, 2004 , 2013 ; Bedouret et al, 2014 ). Moreover, THF clathrate hydrate may be prepared by co-including strong acid molecules (Desmedt et al, 2015 ). Such chemical modification has an impact onto the lattice dynamics of the cages and on the melting points of the hydrates (Desmedt et al, 2015 ).…”

“…Moreover, THF clathrate hydrate may be prepared by co-including strong acid molecules (Desmedt et al, 2015 ). Such chemical modification has an impact onto the lattice dynamics of the cages and on the melting points of the hydrates (Desmedt et al, 2015 ). However, to the best of our knowledge, no studies have been performed until now to investigate the H 2 storage in such THF clathrate hydrates co-including strong acid species.…”