Study On Adsorption Of Hydrogen In Zeolitic Imidazolate Frameworks

At present,the people face the more and more serious energy crisis and environment damage.Hydrogen energy is regarded as the most potential clean energy source to relace the fule,which attracts more and more attentions all over the world. In recent years,Hydrogen economy has been proposed as the blueprint in some developed countries,such as American,European countries,Japan and so on.The problem of hydrogen storage must be solved before it application in real life.Owing to Zeolitic Imidazolate Framework endless varity of structure and functional groups, high accessible surface area and porosity,spatial and finely controlled structure,it have been identified as a category of promising materials for hydrogen storage,and was attracted the researchers focuse from many countries.A number of experiments hydrogen adsorption in ZIFs were published,while most of researches were synthesizing different kind and topology ZIFs,however the adsorption mechanism of hydrogen molecules in ZIFs is still poorly understood.So the studies of adsorption and diffusion behaviors for hydrogen molecules in MOFs are of great instructional significance.Due to the limitations of traditional methods,it is difficult to obtain the multi-component adsorption data by experiments.Molecular simulation techniques, as a kind of"computer experiments",offer an attractive method to help us explain the adsorption phenomena observed macroscopically in the nanoporous materials.Grand canonical ensemble Monte Carlo(GCMC) simulation was performed to study the adsorption behaviors of hydrogen.Furthermore,the mCT(Computer Tomography for materials) technique was developed to investigate the adsorption sites of hydrogen in ZIFs.The adsorption sites and adsorption mechanism of hydrogen molecules in IRMOFs were studied by using mCT methods.The major contributions of this work are as follows: 1.GCMC simulations of hydrogen adsorption on Z1F-8 were carried out to investigate the adsorption isotherms and adsorption sites in ZIFs.A modified OPLS-AA force field was applied for the imidazolate frameworks and Buch's model for hydrogen molecules.The performance of the models and parameters applied in the simulations was evaluated by comparing the calculated results with the experimental data for the adsorption in the case of hydrogen.It shows good performance compared with the experimental measurements.2.GCMC simulations of hydrogen adsorption on ZIF-8 were carried out to investigate the adsorption isotherms and adsorption sites in ZIFs.A modified OPLS-AA force field was applied for the imidazolate frameworks and Buch's model for hydrogen molecules.The performance of the models and parameters applied in the simulations was evaluated by comparing the calculated results with the experimental data for the adsorption in the case of hydrogen.It shows good performance compared with the experimental measurements.3.The hydrogen adsorption sites in the ZIF-8 materials were explored by a previously proposed technique named "computer tomography for materials(mCT)". The mCT images suggest that the first adsorption site locates at both sides of the imidazolate ring and it is close to the imidazolate C=C bond.The hydrogen molecules then are adsorbed in the center of the Zn-hexagonopenning which forms the secondary adsorption site4.When we compared the features of the hydrogen adsorption sites of the ZIFs with the MOFs,some significant differences were found.The steric hindrance effect around the nitrogen atoms and the higher electron affinity of sp~2-hybrid carbon atoms in ZIF-8 lead to hydrogen molecule adsorption close to the C=C bond.The difference between the ZIF's adsorption sites and the MOF's was analyzed,and it may be helpful to design new ZIFs materials with improved hydrogen adsorption capability.5.Inorder to investigate the effects of different metal atoms in ZIFs at the behavior of adsorption hydrogen.ZIF-8,ZIF-67,ZIF-11 and ZIF-12 were selected to compare.At 77K,the hydrogen adsorption in ZIF-8 and ZIF-11 which posses Zn atom is higher than ZIF-67 and ZIF-12 which posses Zn atom respectively.6.The mCT method was employed to investigate the adsorption sites in ZIFs, and the results indicates that the first adsorption sites is still near the C=C double bond after Co atom replace the Zn atom,but the adsorption sites in ZIF-67 are more close to the metal atoms.At last,the comparition of adsorption sites of ZIF-11 and ZIF-12 with the increase of pressure was studied.The major adsorption sites behavior no sign to change.The secondary adsorption sites became less important with pressure increasing as the radiu of the channel.