Adsorption And Diffusion Behaviors Of Fluid Molecules In Microporous Materials

The diffusion and adsorption properties of fluids and its mixtures in microporous media are of great importance in diversified applications. So the subject investigated is of great theoretical significance and application prospects. However, the understanding of adsorption and diffusion properties of confined fluids and its mixtures are still very superficial, and the experimental data are lacking under the extreme conditions. The difficulties and challenges are that conventional theories are most adequate for describing macroscopic phenomena while the dynamics of molecules in small pores is often strongly affected by the interactions between fluid molecules and the confining walls, which leads to the situation becoming even more complicated. The transport coefficients are of great importance in the fields of chemical engineering, materials, medicine, energy sources, environmental protection, and micro chemical reactors. It is necessary to investigate the transport properties from the molecular levels, especially for the mutual diffusion coefficients, which are less investigated. In recent years, hydrogen energy is regarded as the most potential clean energy source, which attracts more and more attentions. Hydrogen economy has been proposed as the blueprint in some developed countries, such as American, European countries, Japan and so on. In order to safely and effectively obtain and make use of the hydrogen energy, more and more fund was invested for investigation. Recently, metal-organic frameworks (MOFs) have been identified as a category of promising materials for hydrogen storage. A number of experimental hydrogen adsorption in MOF materials were reported, while most of these studies focused on synthesizing different kinds and topology MOFs, and the adsorption mechanism of hydrogen molecules in MOFs is still poorly understood. So the studies of adsorption and diffusion behaviors for hydrogen molecules in MOFs are of great theoretical significance and application prospects.First, molecular dynamics simulations were employed to calculate the mutual sdiffusion behaviors of Ar/Kr mixtures and ethanol/water mixtures in the bulk. Then the self- and mutual diffusion coefficients of Ar/Kr mixtures confined in the model nanopores were further investigated through MD simulations. Second, the grand canonical Monte Carlo (GCMC) simulations were employed to investigate the hydrogen adsorption behaviors in IRMOFs. An effective method denoted as 'Computer Tomography for materials (mCT)' was developed to directly view the adsorption sites in any planes and from any angles. The adsorption sites and adsorption mechanism of hydrogen molecules in IRMOFs were studied by using mCT methods. Besides, a hydrogen probe molecule was pushed into the pore channels of IRMOFs to calculate the average interaction energy between adsorbed molecules and frameworks. The most important factor that influence the amount adsorbed was further explored based on these studies. Then the designed rule was proposed, and some new MOF materials with high hydrogen storage capacity were designed based on the rule. In addition, GCMC combined with the configurational-bias Monte Carlo simulation technique was employed to study the adsorption and separation of longer alkane (C₄-C₇) isomer mixtures in IRMOFs. The major contributions of this work are as follows:1. The mutual diffusion coefficients of Ar/Kr mixtures and ethanol/water mixtures in the bulk were investigated via the thermodynamic factor (Q) and the kinematic factor (L). The result indicates that the kinematic factor increases linearly as the mole fractions of Argon (x_Ar) and the distinct diffusion coefficient part for Ar/Kr mixture are closer to zero. For the ethanol/water mixtures system, however, the dependence of the kinematic factor on composition is not obvious. When the molar fraction of ethanol (x_E) is small than 0.3, L decreases as the x_E; when X_E is largerthan 0.3, it fluctuates around a constant. Owing to the poor ideality of ethanol-water mixtures, the distinct diffusion coefficients are obviously larger than zero. However, the change trend for ideal mixtures is controlled by the kinematic factors. Thus the mutual diffusion coefficients could be approximately calculated from the self-diffusion coefficients by L₀ = x_AD_B + x_BD_A . While the mutual diffusion coefficients for non-ideal mixtures are mainly controlled by the thermodynamic factors.2. The diffusivities of Ar-Kr mixtures confined in channel pores at different mole fractions were calculated. The ideality of the mixtures will be slightly changed owing to the confinement of the walls, and the pore width significantly affects the diffusion behaviors of mixtures in the nanopores. Both the self- and mutual diffusivities in nanopores are much lower than that of the bulk, and they decrease as the pore width decreases but increase as the temperature increases.3. The adsorption isotherms for hydrogen in IRMOFs at 77 K and 298 K were calculated with GCMC simulations. At 77K, IRMOFs displays the saturated adsorption as pressure increases to 50 bar. The adsorption capacity of IRMOFs around room temperature is very low, and the hydrogen storage amount is only 2 wt% even pressure up to 100 bar. The mCT method was developed and employed to investigate the adsorption sites in IRMOFs, and the results indicates that the first-adsorption site is near the location of oxygen atoms where three -COO groups joined like a cup, and it denoted as a(-COO)₃. It was found that two adsorption sites located at the diagonal of Zn₄O clusters are in the plane "A", while other two equivalent adsorption sites are in another plane 'B', which is about 5.4 A away from the plane "A". "A-B-B-A" cycle can also be observed. At lower temperatures, as the pressure increases, hydrogen molecules were preferentially adsorbed in the Zn₄O cluster. Then more and more hydrogen molecules were adsorbed around the organic linkers, and finally hydrogen molecules were adsorbed in the pore channels of IRMOFs. The interaction energy calculations via molecular probe further confirm that the Zn₄O cluster plays a much more important role than the organic linker during adsorption; and the closer to the Zn₄O cluster, the stronger interaction between the frameworks and the molecules adsorbed. The repulsive force becomes the dominant interaction between the frameworks and the hydrogen molecule as the probe molecule much closer to the Zn₄O cluster. It was found that the preferential adsorbed sites in IRMOFs less convergent than that at lower temperatures as the temperature increase. The interaction energy of hydrogen molecules in different planes 'A and 'B' reveals that the adsorption sites and the interaction energy were mainly determined by the structure of IRMOF-1.4. It was found that the oxygen atoms play an important role for hydrogen adsorption. On the basis of this viewpoint, the designed rule for MOFs was further proposed, which reveals that the hydrogen storage capacity might be improved by introducing some strong electronegativity atoms to the organic linker. Five new MOFs designed on the basis of this viewpoint by introducing some strong electronegativity atoms (F, Cl) to the organic linker of IRMOF-1. The hydrogen storage capacity for newly designed MOFs is further evaluated by simulations, which is found that the hydrogen adsorption amounts for newly designed MOFs improved remarkable, and the amount for MOF-d5 at 1 bar is as high as 3.7 wt%. It can be observed that extra adsorption sites were formed in the pores and the effective occupation rate of pore space was obviously improved viewing from the mCT images.5. The GCMC combined with the CBMC simulation technique was employed to study the adsorption and separation behavior of longer alkane isomer mixtures (C₄-C₆) in IRMOF-6 and -1. It was found that the amount adsorbed of linear and branched alkanes increases with the pressure increasing in IRMOF-1, -6, and the amount adsorbed of branched alkanes is larger than that of linear ones at higher pressures. The result reveals that IRMOF-1 and IRMOF-6 have close values of the selectivity for C₄-C₆ alkane isomers mixtures, and they are close to unit. The selectivity of IRMOF-6 is slightly better than that of IRMOF-1. The interaction energy calculations via molecular probe further confirm that the Zn₄O cluster plays much more important role than the organic linker during adsorption; and the closer to the Zn₄O cluster, the stronger interaction between the frameworks and the molecules adsorbed. It was observed that the branched alkane molecules were hard to approach the inorganic corner of IRMOF-6 owing to the stronger repulsive interaction caused by the space hindrance between the methyl group and the side groups of linker. In addition, the adsorption selectivity was investigated from the viewpoints of thermodynamics and kinetics. It was found that the adsorption selectivity is mainly controlled by the adsorption enthalpy when the alkane mixtures were adsorbed in the pore channels. On the other hand, it is determined by the adsorption entropy when the alkane molecules are close to the Zn₄O cluster. During molecular dynamics simulation, the number density for n -butane around the Zn₄O cluster of IRMOF-6 is larger than that for 2-methylpropane, and the resident time for one n -butane molecule around the Zn₄O cluster is also slightly longer than that for 2-methylpropane.