CO2 hydrate nucleation kinetics enhanced by an organo-mineral complex formed at the montmorillonite-water interface

Clay nanoflakes and organic molecules synergistically promoting CO2 hydrate formation

Carbon dioxide (CO₂) reduction is an urgent challenge worldwide due to the dramatically increased CO₂ concentration and concomitant environmental problems. Geological CO₂ storage in gas hydrate in marine sediment is a promising and attractive way to mitigate CO₂ emissions owning to its huge storage capability and safety. However, the sluggish kinetics and unclear enhancing mechanisms of CO₂ hydrate formation limit the practical application of hydrate-based CO₂ storage technologies. Here, we used vermiculite nanoflakes (VMNs) and methionine (Met) to investigate the synergistic promotion of natural clay surface and organic matter on CO₂ hydrate formation kinetics. Induction time and t₉₀ in VMNs dispersion with Met were shorter by one to two orders of magnitude than Met solution and VMNs dispersion. Besides, CO₂ hydrate formation kinetics showed significant concentration-dependence on both Met and VMNs. The side chains of Met can promote CO₂ hydrate formation by inducing water molecules to form a clathrate-like structure. However, when Met concentration exceeded 3.0 mg/mL, the critical amount of ammonium ions from dissociated Met distorted the ordered structure of water molecules, inhibiting CO₂ hydrate formation. Negatively charged VMNs can attenuate this inhibition by adsorbing ammonium ions in VMNs dispersion. This work sheds light on the formation mechanism of CO₂ hydrate in the presence of clay and organic matter which are the indispensable constituents of marine sediments, also contributes to the practical application of hydrate-based CO₂ storage technologies.

Organics-Coated Nanoclays Further Promote Hydrate Formation Kinetics

A deeper understanding of the kinetics of CO₂ hydrate formation in the complicated natural environment is required for its enhanced sequestration. Here we found that the organics-coated nanoclays enriched in the natural sediments could contribute to a 92% decline of the induction time of hydrate formation. This can be ascribed to the negative charges carried by the organics and the resulting ordered arrangement of the surrounding water molecules. It was, for the first time, proposed that the abundant functional groups from the coating organics could function as a protecting crust enabling the system more resistant to the acidification potentially upon the CO₂ sequestration; besides, the negative charges could help prevent the deposition of the nanoclays via interparticle repulsive forces. These would consequently secure their sustainable promoting effect on hydrate formation. The findings suggest the deposits of gas hydrate a kinetically promising geological setting for the CO₂ sequestration via forming hydrates.

Effects of Layer-Charge Distribution of 2:1 Clay Minerals on Methane Hydrate Formation: A Molecular Dynamics Simulation Study

Molecular dynamics simulations were used to investigate the effects of the external surface of a 2:1 clay mineral with different charge amounts and charge locations on CH₄ hydrate formation. The results showed that 5¹², 5¹²6², 5¹²6³, and 5¹²6⁴ were formed away from the clay mineral surface. The surface of the clay mineral with high- and low-charge layers was occupied by Na⁺ to form various distributions of outer- and inner-sphere hydration structures, respectively. The adsorbed Na⁺ on the high-charge layer surface reduced the H₂O activity by disturbing the hydrogen bond network, resulting in low tetrahedral arrangement of H₂O molecules near the layer surface, which inhibited CH₄ hydrate formation. However, more CH₄ molecules were adsorbed onto the vacancy in the Si-O rings of a neutral-charge layer to form semicage structures. Thus, the order parameter of H₂O molecules near this surface indicated that the arrangement of H₂O molecules resulted in a more optimal tetrahedral structure for CH₄ hydrate formation than that near the negatively charged layer surface. Different nucleation mechanisms of the CH₄ hydrate on external surfaces of clay mineral models were observed. For clay minerals with negatively charged layers (i.e., high and low charge), the homogeneous nucleation of the CH₄ hydrate occurred away from the surface. For a clay mineral with a neutral-charge layer, the CH₄ hydrate could nucleate either in the bulk-like solution homogeneously or at the clay mineral-H₂O interface heterogeneously.

Nucleation Curve of Carbon Dioxide Hydrate from a Linear Cooling Ramp Method

Formation of gas hydrate is a first-order phase transition that starts with nucleation. Understanding of nucleation is of interest to many in chemical and petroleum industries, as nucleation, while beneficial in many chemical processes, is detrimental in flow assurance of oil and natural gas pipelines. A primary difficulty in the investigation of gas hydrate nucleation has been the inability of researchers to compare nucleation rates of gas hydrates across various systems of different scales and complexities, which in turn has been limiting the ability of researchers to study the nucleation process itself. In this study, a first-generation high-pressure automated lag time apparatus (HP-ALTA MkI) was used to determine the nucleation curve of structure I (sI) - forming carbon dioxide hydrate. The instrument subjected a quiescent water sample of well-defined dimensions to a large number of linear cooling ramps under isobaric conditions, and detected and recorded carbon dioxide hydrate formation temperature distributions. A survival curve was constructed from the measured ensemble, and a nucleation curve was derived from the survival curve using the empirical model-independent method we had previously reported. The nucleation rate of carbon dioxide hydrate was found to be significantly greater than that of pure methane hydrate or that of natural gas hydrate over the entire range of subcooling investigated. We provide a new physical interpretation of an experimentally determined nucleation curve and, by doing so, solve one of the outstanding puzzles of the HP-ALTA technology.

Formation of Methane Hydrate in the Presence of Natural and Synthetic Nanoparticles

Natural gas hydrates occur widely on the ocean-bed and in permafrost regions, and have potential as an untapped energy resource. Their formation and growth, however, poses major problems for the energy sector due to their tendency to block oil and gas pipelines, whereas their melting is viewed as a potential contributor to climate change. Although recent advances have been made in understanding bulk methane hydrate formation, the effect of impurity particles, which are always present under conditions relevant to industry and the environment, remains an open question. Here we present results from neutron scattering experiments and molecular dynamics simulations that show that the formation of methane hydrate is insensitive to the addition of a wide range of impurity particles. Our analysis shows that this is due to the different chemical natures of methane and water, with methane generally excluded from the volume surrounding the nanoparticles. This has important consequences for our understanding of the mechanism of hydrate nucleation and the design of new inhibitor molecules.

Highly porous CO2 hydrate generation aided by silica nanoparticles for potential secure storage of CO2 and desalination

We report a new way of storing CO₂ in a highly porous hydrate structure, stabilized by silica nanoparticles (NPs).