Molecular Resolution Into The Nucleation And Crystal Growth Of S? Clathrate Hydrates

Energy materials provide the basic motive power for the world economic developing process,it is not only having a relationship with people's daily life,but also affects the stability for a given regions.With the rapidly growing of human society,the global demand of energy resources is rising.While the environmental problems induced by the continuously consumption of non-renewable energy resources cannot be overlooked.In order to suppress the global warming,China will work hard for reaching the peak of carbon emissions before and strive to achieve carbon neutrality around 2060.Gas hydrates as a mineral resource can provide vast clean energy,and innovative application of hydrate-based technologies can promote the trans-formation process of green and lowcarbon development mode in many aspects,for example CO2 capture,gas strong and separation,and seawater desalination.A clear and thorough molecular-scale understanding of hydrate formation is essential for the development of innovative hydrate-based applications safely and high-efficiently.While the nucleation and crystal growth mechanism of clathrate hydrates is not fully resolved at the molecular level due to the stochastic nature and the inaccessible small length and time scale for experimental investigations.Even though significant knowledge has been gained through performing Molecular Dynamics simulation over the past several years,due to the limited number of systems and conditions that have been considered in previous MD simulation studies,the present understanding of the clathrate hydrate formation mechanism still represents just the tip of the iceberg.Gas hydrates have diverse crystal structures,and the Structure ? is the most prevalent in numerous applications including gas production,gas storage,and seawater desalination.Previous studies on the nucleation of hydrates focused on the guest molecules that forms s I hydrates.Despite the importance of s ? hydrates,less focus has been given to understand the molecular mechanism for the nucleation of s ? hydrates.Many unanswered questions still exist at molecular level to understand the nucleation and crystal growth process of s ? hydrates.This thesis focus on the guest molecule that can form s ? hydrates and investigate the nucleation and crystal growth process at molecular level.In particular,two set of systemic MD simulations were performed,that is,(1)pure propane hydrates and(2)methane/propane mixed hydrates.We proposed a novel method,the Largest Cage Cluster Order Parameter(LCC-OP),to track the nucleation of hydrates and track the evolution of crystal structure in the growing solid.The main results are summarized below.This thesis successfully captured the homogeneous nucleation of propane hydrates,and it is the first report on the propane hydrates from a disordered two phase-separated system.The formation process of the empty cage is elucidated at molecular level for the first time.For pure propane hydrates,the empty cages present much more frequently than propane occupied cages during prenucleation period,however,an equal occurrence of empty and occupied cages is observed and needed for the growth of the LCC,and this about-equal number ratio is retained in the hydrate growth stage.Using the LCC as the order parameter,a critical nucleus size of eight cages was obtained for propane hydrates at 250 K and 180 MPa.The critical nucleus is diverse in terms of the structure and composition,indicating that there are multiple pathways leading to nucleation.In general,the critical nucleus tends to have an equal number of empty and occupied cages,which also emphasizes the importance of both empty and occupied cages.The evolution of other separated clusters smaller than the largest one is also tracked,giving direct insight into the competition of different clusters in the prenucleation,nucleation,and survival of the LCC in the growth,while the smaller clusters remain subcritical in size.We also performed 20 s annealing simulations.Despite the transformation of propane occupied cages from amorphous structure toward the elementary cage of the s ? crystal,however,the propane hydrate solid remains amorphous at the end of annealing simulation.The annealing from the amorphous solid to a crystalline structure happens very slowly such that 20 s is still not long enough to capture long-range ordering resembling the s ? crystal.In terms of methane/propane mixed hydrates,this thesis performed systematic microsecond MD simulations with different initial methane/propane content to investigate the spontaneous homogeneous nucleation and growth of methane/propane mixed hydrates from two-phase separated systems of water and methane/propane mixtures.Results show that the difference in the uptake kinetics between methane and propane into the solid phase,and the enclathration rate of free methane molecules into the clathrate phase is higher than that of propane molecules.This study demonstrates the uncorrelations between the solution compositions and the occupancy in the growing solid clathrate phase.The critical nucleus size,obtained from the probability of growth and decay of a given cluster size,is consistent among the three systems compared,ranging from 7 to 9 cages.With the increase of methane content in the system,the range of the critical nucleus becomes broader,and the composition of the critical nucleus changed a lot in terms of cage geometry and occupancy.In the system with a high methane content in the initial mixture,the critical nucleus showed features closer to type I than type ? clathrate.A new method is proposed to measure and track the evolution of sI and s? pattern clusters,providing direct evidence that the crystal structures can germinate from the amorphous hydrate-like solid,and amorphous cages around the crystal seeds can transform to elementary cages of crystalline strucutres.Coexistence of s I and s ? patterns was observed and tracked in a high methane content system.Due to the solid-phase faster uptake of methane molecules than propane molecules,the s I patterns growing earlier and faster than s ? patterns in the early stage of solid growth,however,the degree of crystallinity of s ? is obviously greater than that of s I.With more propane entering the solid phase,s ? patterns manifested as the eventual stable structure for methane/propane mixed hydrates.In the pure propane hydrate study,we did not capture s ? patterns even after 20 ? s annealing simulations,while the s ? patterns are captured in almost all nucleating trajectories for the methane/propane mixed systems,indicating that the introduction of methane catalyzes the formation of s ? crystals.In this thesis,two guest molecules system that can form s ? hydrates are detailed investigated via Molecular Dynamic simulations.The molecular details revealed here advance our understanding of the nucleation and crystal growth of clathrate structures.Moreover,the new methods and novel order parameters proposed in this study to track the nucleation and track the evolution of crystal structures further improve the existing knowledge system,which can provide theoretical guidance for efficient and innovative application of gas hydrate.