COMPUTATIONAL STUDIES OF SELECTED GAS HYDRATES AND HYDRATE MELTS

Abstract

Gas hydrates are non-stoichiometric crystalline compounds formed when water and small gas molecules are brought together at low temperature and high pressure. Under these conditions, gas molecules are trapped inside the cages formed by water molecules. Gas hydrates are stabilized by the hydrogen bonding interactions between water molecules as well as by the van der Waals interaction between the guest molecules and the host cages. Depending on the size and shape of the guest molecules, gas hydrates occur in different structural forms. The amount of hydrocarbons that can be obtained from natural gas hydrates is reported to be twice the amount that is available from conventional fossil fuels. The natural gas hydrates contain mainly methane and hence they are known as methane hydrates. There are reports that the amount of hydrocarbons that can be obtained from natural gas hydrates is twice the amount that is available from conventional fossil fuels. Gas hydrates can also be used as transportation and storage materials for various gases. However, a prior knowledge on host-guest and guest-guest interactions in gas hydrates is important for their efficient use, as it provides valuable information about stability of the hydrates under various conditions. Most of the studies in this direction are limited to the host-guest interaction in single cages. Thus, in the present thesis, an attempt has been made to give a detailed picture of such interactions by incorporating more such cages and guest species. The thesis is divided into seven chapters. Different types of natural gas hydrates, and their applications are briefly discussed in the first chapter. The discussion on gas hydrates is followed by a literature review on gas hydrates of different molecules, their stability and various spectroscopic properties. The second chapter focuses on the computational methodology used in the present work. Various quantum mechanical methods such as Hartree-Fock, post Hartree-Fock and density functional methods are briefly discussed in this chapter. It also covers the details on basis sets and their classifications. A description on classical molecular dynamics methods along with various types of interaction potentials and force fields commonly used in simulations is presented. The strategies adopted for modeling water cages of different size and shape are also explained. The above strategies based on the concept of strong-weak-effective-bond (SWEB) model is extended for generating initial ii geometries of fused water cages with maximum number of strong t1d hydrogen bonds. The optimized geometries of these cages are further used for encapsulating various gas molecules. Taking noble gas atoms as prototypes, the effect of size of guest species on the stability of their hydrates is investigated in chapter 3. For this purpose, different noble gas atoms are encapsulated in the cavities of dodecahedral, fused dodecahedral and triple-fused dodecahedral water cages and are studied by using dispersion corrected density functional method B97D in conjunction with cc-pVTZ basis set. The results showed that the size of guest species plays an important role in the host-guest interaction. Among the guest species He, Ne, Ar and Kr, interaction energy varied in the order: He@DD < Ne@DD < Ar@DD < Kr@DD and is in agreement with the earlier reports. The study on the interactions of a noble gas atom with the neighboring cavity as well as its interactions with the guest species residing in the neighboring cage indicated that such interactions are not significant for small guest species. The values of interaction energy obtained for the mixed noble gas hydrates indicated that the presence of a guest in one of the cavities favors a guest atom of larger size in the neighboring cavity. It is also revealed that the changes in enthalpy and Gibbs free energy associated with the encapsulation of noble gas atoms are independent of the presence of a neighboring cage or the guest atoms present in that. In the fourth chapter, the encapsulations of diatomic (H2), triatomic (CO2) and polyatomic (CH4) molecules which differ in both size and shape are investigated at B97D/cc-pVTZ level of theory. A variety of fused cages formed by the combinations of dodecahedral and irregular dodecahedral water cages is used for this purpose. The host-guest interactions are studied and the results are analyzed in terms of interaction energy and interaction energy per guest molecule. A comparison of the interaction energy obtained for a molecule trapped in a single cage with that obtained for various fused cages suggested that the interaction of small guest species does not extend beyond a cage. However, the interaction of large guest species is not limited to the cage where it is located; rather it extends to the neighboring cage thereby interacting with the guest species in those cages. The vibrational Raman stretching frequencies of the guest molecules confined in various types of cages are computed and analyzed. The results showed that the stretching frequencies are increased inside the cages due to confinement. However, such shifts decreased with an increase in the size of the cavity. The studies also revealed that neither an adjacent cage nor the guest species in that cage influences the stretching frequencies of a molecule encapsulated. iii The nuclear magnetic resonance (NMR) chemical shifts for 1H and 13C nuclei of the guest molecules in their free and encapsulated states are also computed using multi reference standard method. The results indicated that both 1H and 13C nuclei of the guest species undergo deshielding on their encapsulation. The analysis also revealed that such deshielding is decreased with an increase in the size of the cavity. The study did not show any significant change in the NMR characteristics of an encapsulated molecule in presence of an adjacent cage or a guest molecule inside. Considering the potential of gas hydrates as hydrogen storage materials, the structure, stability and properties of mixed hydrates of hydrogen and tetrahydrofuran are investigated in chapter 5 using B97D/cc-pVTZ level of theory. Multiple encapsulations of hydrogen molecules in two different types of water cages are studied to find out the optimum occupancy of the cages with and without the presence of tetrahydrofuran. The change in host-guest interaction energy due to the successive addition of H2 molecules in dodecahedral and hexakaidecahedral water cages is investigated. The results suggested that H2 molecules can be encapsulated along with tetrahydrofuran in hexakaidecahedral cage of SII hydrates, thereby increasing the hydrogen storage capacity. The thermo-chemical parameters obtained are also found to be in agreement with the above findings. The 1H and 13C chemical shifts values of H2 and THF molecules are computed to infer about the host cages in which the guest species are confined. The effect of neighboring water cages on the above parameters is also discussed. It is observed that NMR chemical shift values of H2 are deshielded on encapsulation and the presence of THF in a neighboring cage has little influence on the chemical shift. Considering the potential of gas hydrates as energy resource, several methods have been proposed and explored in the past to extract natural gas from hydrate sediments. Among these methods, the replacement of CH4 by CO2 has received wide attention due to its potential to sequestrate CO2 along with the extraction of CH4. However, the yield of methane recovery by this method has been reported to be 60-64%. It has been proposed that a mixture of N2 and CO2 can significantly improve the percentage of methane recovery from its hydrates. During the replacement of CH4 by a mixture of CO2 and N2, the intermediate liquid phase formed contains CH4, N2 and CO2 dissolved in it. The evolution of dissolved gas molecules in this liquid is expected to significantly influence the subsequent steps of extraction process. Thus, in the sixth chapter, the evolution of dissolved gas molecules from a mixture of CH4, CO2, N2 and H2O is studied by applying classical molecular dynamics simulations. iv The study revealed that an increase in the concentration of CO2 assists the formation of nanobubbles in a CH4-N2-CO2-H2O mixture. The composition of the bubbles formed was analyzed and it revealed that the bubbles are made of CH4, N2 and CO2 molecules. To understand the role of CO2 in assisting the formation of nanobubbles in the mixture, the distribution of gas molecules in the bubble was examined. It was found that CO2 molecules accumulate near the surface of nanobubbles and become more prominent with an increase in the concentration of CO2 in the mixture. The CO2 molecules at the surface of the bubble reduced the excess pressure inside the bubble as well the surface tension at the bubble-water interface thereby assisting the bubble nucleation. To understand the effect of nanobubbles on hydrate nucleation in the mixture, the structural ordering of water molecules around the bubble is investigated. The number of rings formed by water molecules around the bubble in the CH4-N2-CO2-H2O mixture is analyzed. It is observed that water rings are formed preferentially near the surface of the bubble. The number of water rings formed per unit volume near the bubble-water interface is correlated to the dynamic nature of the bubbles. The nanobubbles which are more dynamic have a larger number of water rings formed per unit volume near its surface compared to the less dynamic ones suggesting the possibility of gas hydrate nucleation near the bubbles.