1.INTRODUCTION
The 21st century is the era when the traditional fossil energy is being replaced by clean energy.1, 2 Clathrate hydrates (hereinafter referred to as “hydrates” when no misleading) are non-stoichiometric crystalline inclusion compounds that can form under low temperatures or high pressures conditions and can exist both above and below the freezing point of water.3, 4 More than 130 guest compounds are known to form hydrates with water molecules and are typically hydrophobic natural gases such as CH4 and CO2.5 Hydrates are of interest to the public due to the well known natural gas hydrates. Their formation requires relatively low temperature and high pressure.1 There are three common types of gas hydrate structures: sI, sII, and sH hydrates. In the environment, gas hydrates are mainly of the sI and sII types. sH hydrates are also confirmed to exist in nature, such as in the Gulf of Mexico and Cascadia Margin.6-8 Although other structural types such as sT type and half-clathrate hydrates were also reported,9-11 so far they exist only in the laboratory. In the mid 1960s and early 1980s large reservoirs of gas hydrates were found in permafrost and marine areas, respectively,12, 13 by governments, as well as oil and gas companies. This raised the interest in exploitation and application related activities from all walks of life, especially the academic community. For hydrates as an energy resource, their stability needs to be addressed thoroughly for the correspondence to global climate change since methane is an important greenhouse gas. Therefore, a growing number of investigations on natural gas hydrates have been conducted, and the research contents have expanded from the initial flow assurance for preventing blocking of oil and gas pipelines14, 15to resource potential,16-18 safe drilling,19 geological hazards,20, 21 the carbon cycle,22 climatic change,23, 24 and even outer space hydrates.25
Due to the massive emissions of greenhouse gases that have caused the global warming, scholars have proposed the capture and storage (CCS) of greenhouse gases.26 Under this storage technology, the replacement of CH4 with CO2 from natural gas hydrates has been demonstrated to be possible from both experimental and theoretical investigations. Considering the complication in geology as well as exploitation conditions for the practice of CCS with natural gas hydrates, it is necessary to determine the elasticity and mechanical strength of the generated greenhouse gas hydrate membrane.27 Moreover, this also provides requisite data in future hydrate technology to design the structures, shapes and sizes of the transport systems according to the mechanical strength of solid hydrate.28 Up to now, exploration on hydrate mining and replacement is concentrated to CO2 and organic gases, and only limited works have been carried out on the remaining greenhouse gases, which were found have the potential to extract natural gas from hydrates in the previous work.29 Therefore, it is crucial to find the mechanical properties for different greenhouse gas components, such as nitrogen-containing small molecules, encapsulated in clathrate hydrates.
With the development of super computing technology, theoretical calculations have been recognized as a powerful scheme that can provide critical insight and understanding of the structure and properties of gas hydrates. In recent decades, several properties of gas hydrates have been predicted in theory including the structures, thermodynamic stability, nucleation and growth processes, grain size and grain mechanics. For example, Rey et al. used density functional theory (DFT) to study the mechanical properties and structures of methane and carbon dioxide hydrate, which shows that the two gas hydrates are both highly isotropic, but they differ significantly in shear properties.30 English et al.carried out molecular dynamics (MD) simulations to analyze the dynamic properties of sI H2S hydrate.31Zeina et al calculated the elastic mechanical parameters of methane hydrate using the first principle method, and the obtained results agree well with the previous experimental data32. Uchida et al. used MD simulation to calculate the bulk elastic modulus of hydrates in different ratios of CH4 and CO2.33, 34 In our previous works, we have studied the structure and formation mechanisms of clathrate hydrates encaging different gases34 and the structures and mechanical properties of CH4, SO2, and H2S hydrates were investigated with DFT.35 Recently, the occupancy isotherms of pure CH4, pure CO2 and their mixture in sI and sII hydrates are studied by our group using grand canonical Monte Carlo and molecular dynamics hybrid (GCMC+MD) simulations. We have shown that the mixing of CO2 into CH4 may have stabilization effect on sI hydrate, providing a thermodynamic basis for the feasibility of CO2 to promote CH4mining.36 Then the adsorption behavior and phase equilibrium for clathrate hydrates of sulfur- and nitrogen-containing small molecules were also predicted. We found that the SO2, H2S, N2O, and even CS2 gases have the ability to replace the CH4 gas from natural gas hydrates. Results from that study suggest that some components in flue gases may assist the displacement of CH4. This implies that one may save significant effort in separation of different components from flue gases when performing replacement of CH4 gas in natural gas hydrates with CO2.29, 36
Up to now, investigations on structures and properties for some hydrates like CO2, H2S and SO2hydrates35, 37 have been performed, but only a few inspections have been conducted on mechanical properties of hydrates encaging nitrogen-containing small molecules. This article presents a computational study on the elastic constant tensor of nitrogen-containing small molecules hydrates from DFT. This work seeks to shed a light on the differences in elastic mechanical properties of three hydrates and their impact on hydrates’ behavior.