Theoretical Study On Stability Of Clathrate Hydrates Encapsulated With Guest Molecules

Gas hydrates are an ice-like non-stoichiometric compound consisting of water molecules surrounding and trapping gas particles.Hydrogen-bonding formed between water molecules holds this lattice together in a crystalline structure.In nature,there are amount of gas hydrates,which is estimated to be more than two times than the total fossil fuel available.On one side,the large amount of natural gas hydrates present in nature is considered to be a future clean energy source,which can be used instead of the traditional energy source to solve imminent energy crisis.On the other side,gas hydrates have characteristics such as high gas storage capacity,gas trapping selectivity,which make them promising for applications in natural gas storage,gas separation.It is difficult to accurately predict the nucleation/dissociation mechanism of clathrate hydrates under present experimental conditions.Thus,using theoretical research methods to study gas hydrate at molecular and atomic level,which can provide theoretical basis for the basic physicochemical properties,nucleation mechanism and stability of hydrate.In this paper,the stability of three cavity types 512?51262?51264 and double-cage512/51262 of clathrate hydrates are studied by using the density functional theory(DFT)in the gaussian09 package.Firstly,the geometry structures were optimized at B3LYP/6-31+G(d)level,and the corresponding vibrational frequencies were also calculated that the same level,and the basic structural parameters of each structure were obtained to determine the influence of guest molecules on the size of cage structure.Secondly,on the basis of the optimized geometry,single-point energy calculation was performed at M06-2X/6-31++G(d)level to determine the interaction,stability and hydrogen energies of cage structures.The relative energies of inclusion complexes were investigated to infer the effect of the guest molecules on the stability of the structures.The study results indicate that the guest molecules enhance the hydrogen bondbetween the water molecules in the cage,and the hydrogen energy increase.For cage512/51262,only one guest molecule is added to one of the cages,resulting in the slight expansion of this cage.In comparison,another empty cage structure does not change.With the increase of the number of water molecules,the stability of the cage structure is gradually enhanced.For the three cavity types and partially occupied double-cage,with the increase of molecular size of guest molecules,the stability of hydrates increases and the interaction between guest molecule and water molecules increases.