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.