流体分子在微孔材料中吸附与扩散行为研究
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摘要
多孔材料特别是纳米尺度的微孔材料具有广泛的应用前景，对纳孔材料中流体分子及其混合物的扩散、吸附行为的研究具有重要的理论意义和应用价值。由于受到空间的限制，它们在微孔中的吸附、扩散性质往往难以直接通过实验测定获得。传统的理论往往只适用于描述宏观的现象，对受限流体行为的实验与理论研究一直面临着较大的困难和挑战。研究微孔中流体的传递性质，是化工、材料、医药、能源、环保及多种微装置研究和设计中必不可少的重要数据，因此非常有必要从分子层面上研究微孔介质中流体分子传递性质，特别是研究甚少的互扩散行为。近年来氢能经济(hydrogen economy)己被美、欧、日等发达国家提到了发展日程上，各国纷纷投巨资开展研究开发，以期安全、高效地获取、储存和利用氢能。金属有机骨架微孔材料(metal-organic frameworks，MOFs)被认为是最有前途的储氢材料之一，受到各国研究者的广泛关注。当前有关金属-有机骨架材料的研究多数集中在合成不同种类和拓扑结构的MOF材料，人们对氢气在MOFs中的吸附机制尚不清楚，因此氢气在MOF中的吸附、扩散、特别是吸附机理和氢气分子运动行为的研究，具有重要的理论意义和应用前景。
本论文首先采用分子动力学模拟(molecular dynamics simulation，MD)计算了体相下Ar／Kr混合物以及较复杂的乙醇／水混合物的互扩散行为，并在此基础上研究了受限于模型孔道中Ar／Kr混合物的自扩散以及互扩散系数。在研究了模型微孔的基础上，进一步运用巨正则蒙特卡罗方法(grand canonical MonteCarlo，GCMC)研究了氢气在被广泛认为的最有前途的新型储氢材料IRMOF中的吸附行为，发展材料断层成像的方法(Computer Tomography for materials，mCT)研究气体分子在微孔材料中任意平面上的吸附位点，利用该方法分析了氢气IRMOFs骨架结构中的吸附位点和机制，并利用分子探针的方法进一步计算了氢气分子与IRMOFs骨架结构之间的相互作用能。在这些研究基础上探讨了影响氢气在MOFs中储氢量的主要因素，提出了设计规则，依据设计规则设计了几种新型的具有较高储氢性能的MOF材料。此外，本论文还采用构型导向蒙特卡罗方法(configurational-bias Monte Carlo，CBMC)方法和GCMC相结合的方法探讨了IRMOF材料对C_4-C_6烷烃异构体混合物的吸附和分离性能。本文主要研究结果如下：
1．从热力学影响因素和动力学影响因素两个方面研究了Ar／Kr混合物以及乙醇／水混合物的互扩散系数。研究表明对于Ar／Kr混合物体系，动力学因子L随x_(Ar)增加呈线性增长，其相异扩散系数(distinct diffusivity)接近于零，而对于乙醇-水混合物体系，其L随x_E变化规律不明，当x_E<0.3时，L随x_E的增加而降低，当x_E>0．3时，L在一固定值上下波动，相异扩散系数明显大于零。理想混合物的互扩散系数变化趋势主要由动力学因素所决定，可以通过计算各组分自扩散系数的贡献L_0=x_AD_B+x_BD_A近似获得，而对非理想溶液混合物的互扩散系数变化趋势主要由热力学因子Q所控制。
2．研究了不同浓度下Ar／Kr混合物在通道型微孔中的自扩散和互扩散系数，孔壁的限制使得混合物的非理想性有所改变。孔径大小显著地影响了微孔中混合物的扩散性质，微孔中自扩散和互扩散系数均比体相中的值要小，并且它们随着孔径的减小而减小，随着温度的升高而增大。
3．采用GCMC模拟了77 K和298 K下氢气在IRMOFs中的吸附行为。77 K下当压力到达50bar时，氢气在IRMOFs中基本达到饱和吸附。而在298 K下氢气在IRMOFs当压力高达100bar时，最高吸附量仅达到2wt％。发展了mCT方法研究氢气在IRMOFs中的吸附位点，研究表明，氢气在IRMOFs中的第一吸附位点位于由三个对苯二甲酸中六个氧原子所形成的碗状结构中，此吸附位点称之为：a(COO)_3。其中的两个吸附位点位于Zn_4O簇四面体的对角面所在平面“A”，而另外两个同等的吸附位点则位于另一平面“B”上，两个平面相距5．4 (?)，且两个平面之间遵循“A-B-B-A”的循环。低温下随着压力的变化，氢气分子首先吸附到Zn_4O簇周围，然后再吸附到有机配体周围，最后才吸附在MOF的孔道中。随着温度的不断升高，氢气分子在IRMOFs中的主要吸附位点不及在低温下的集中。氢气分子与IRMOF-1结构中Zn_4O团簇之间的相互作用能大于其与有机配体之间的作用能，且氢气分子越靠近Zn_4O团簇相互作用能越强，当氢气分子非常靠近MOF骨架结构时相互作用能由吸引能转变为排斥能，氢气分子处于不同的平面时其相互作用能由结构所决定。
4．IRMOF结构中的Zn_4O团簇中的氧原子与氢分子相互作用的大小对氢气的吸附量影响最为显著，据此我们提出了分子设计规则：向有机配体中引入电负性大的原子。根据设计规则向IRMOF-1的有机配体对苯二甲酸中引入F，Cl等具有较大电负性的原子，设计了五种新的MOF材料，并对所设计材料的储氢性能进行评估，研究发现所设计的材料储氢性能均有一定程度的的提高，其中MOF-d5在77K和1bar时储氢量高达3．7wt％。利用mCT方法研究了氢气在新设计MOF材料中吸附位点，由于向新设计的MOF材料中引入电负性较大的原子，在新设计的MOF材料孔道中以及有机配体周围产生了一些新的吸附位点。
5．采用CBMC和GCMC相结合的方法研究了IRMOF-6和-1对C_4-C_6烷烃异构体混合物的吸附和分离性能。直链烷烃和支链烷烃在IRMOF结构中的吸附量随着压力的增加而增加，高压下支链烷烃的吸附量大于直链烷烃。IRMOF-1和IRMOF-6对C_4-C_6烷烃异构体混合物的相对选择性均不佳，IRMOF-6的选择性略好于IRMOF-1。烷烃分子和IRMOF骨架结构之间的相互作用能的研究表明烷烃分子与Zn_4O团簇之间的相互作用能大于其与有机配体之间的作用能，且烷烃分子越靠近Zn_4O团簇相互作用能越强。由于甲基和有机配体之间的空间位阻所引起的排斥能使得支链烷烃分子很难靠近IRMOF-6的Zn_4O团簇。并分别从热力学和动力学角度解释了烷烃异构体分子在IRMOF-6中的择位吸附现象，研究表明当烷烃异构体混合物主要吸附在孔道之中时，其吸附选择性主要由吸附焓所决定，反之当它们靠近Zn_4O团簇时，吸附选择性则主要由吸附熵所决定。动力学模拟过程中，正丁烷在IRMOF-6的Zn_4O团簇周围的密度和滞留时间略大于异丁烷。
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 (C_4-C_7) 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 L_0 = 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 Zn_4O 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 Zn_4O 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 Zn_4O cluster plays a much more important role than the organic linker during adsorption; and the closer to the Zn_4O 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 Zn_4O 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 (C_4-C_6) 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 C_4-C_6 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 Zn_4O cluster plays much more important role than the organic linker during adsorption; and the closer to the Zn_4O 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 Zn_4O cluster. During molecular dynamics simulation, the number density for n -butane around the Zn_4O cluster of IRMOF-6 is larger than that for 2-methylpropane, and the resident time for one n -butane molecule around the Zn_4O cluster is also slightly longer than that for 2-methylpropane.
本论文首先采用分子动力学模拟(molecular dynamics simulation，MD)计算了体相下Ar／Kr混合物以及较复杂的乙醇／水混合物的互扩散行为，并在此基础上研究了受限于模型孔道中Ar／Kr混合物的自扩散以及互扩散系数。在研究了模型微孔的基础上，进一步运用巨正则蒙特卡罗方法(grand canonical MonteCarlo，GCMC)研究了氢气在被广泛认为的最有前途的新型储氢材料IRMOF中的吸附行为，发展材料断层成像的方法(Computer Tomography for materials，mCT)研究气体分子在微孔材料中任意平面上的吸附位点，利用该方法分析了氢气IRMOFs骨架结构中的吸附位点和机制，并利用分子探针的方法进一步计算了氢气分子与IRMOFs骨架结构之间的相互作用能。在这些研究基础上探讨了影响氢气在MOFs中储氢量的主要因素，提出了设计规则，依据设计规则设计了几种新型的具有较高储氢性能的MOF材料。此外，本论文还采用构型导向蒙特卡罗方法(configurational-bias Monte Carlo，CBMC)方法和GCMC相结合的方法探讨了IRMOF材料对C_4-C_6烷烃异构体混合物的吸附和分离性能。本文主要研究结果如下：
1．从热力学影响因素和动力学影响因素两个方面研究了Ar／Kr混合物以及乙醇／水混合物的互扩散系数。研究表明对于Ar／Kr混合物体系，动力学因子L随x_(Ar)增加呈线性增长，其相异扩散系数(distinct diffusivity)接近于零，而对于乙醇-水混合物体系，其L随x_E变化规律不明，当x_E<0.3时，L随x_E的增加而降低，当x_E>0．3时，L在一固定值上下波动，相异扩散系数明显大于零。理想混合物的互扩散系数变化趋势主要由动力学因素所决定，可以通过计算各组分自扩散系数的贡献L_0=x_AD_B+x_BD_A近似获得，而对非理想溶液混合物的互扩散系数变化趋势主要由热力学因子Q所控制。
2．研究了不同浓度下Ar／Kr混合物在通道型微孔中的自扩散和互扩散系数，孔壁的限制使得混合物的非理想性有所改变。孔径大小显著地影响了微孔中混合物的扩散性质，微孔中自扩散和互扩散系数均比体相中的值要小，并且它们随着孔径的减小而减小，随着温度的升高而增大。
3．采用GCMC模拟了77 K和298 K下氢气在IRMOFs中的吸附行为。77 K下当压力到达50bar时，氢气在IRMOFs中基本达到饱和吸附。而在298 K下氢气在IRMOFs当压力高达100bar时，最高吸附量仅达到2wt％。发展了mCT方法研究氢气在IRMOFs中的吸附位点，研究表明，氢气在IRMOFs中的第一吸附位点位于由三个对苯二甲酸中六个氧原子所形成的碗状结构中，此吸附位点称之为：a(COO)_3。其中的两个吸附位点位于Zn_4O簇四面体的对角面所在平面“A”，而另外两个同等的吸附位点则位于另一平面“B”上，两个平面相距5．4 (?)，且两个平面之间遵循“A-B-B-A”的循环。低温下随着压力的变化，氢气分子首先吸附到Zn_4O簇周围，然后再吸附到有机配体周围，最后才吸附在MOF的孔道中。随着温度的不断升高，氢气分子在IRMOFs中的主要吸附位点不及在低温下的集中。氢气分子与IRMOF-1结构中Zn_4O团簇之间的相互作用能大于其与有机配体之间的作用能，且氢气分子越靠近Zn_4O团簇相互作用能越强，当氢气分子非常靠近MOF骨架结构时相互作用能由吸引能转变为排斥能，氢气分子处于不同的平面时其相互作用能由结构所决定。
4．IRMOF结构中的Zn_4O团簇中的氧原子与氢分子相互作用的大小对氢气的吸附量影响最为显著，据此我们提出了分子设计规则：向有机配体中引入电负性大的原子。根据设计规则向IRMOF-1的有机配体对苯二甲酸中引入F，Cl等具有较大电负性的原子，设计了五种新的MOF材料，并对所设计材料的储氢性能进行评估，研究发现所设计的材料储氢性能均有一定程度的的提高，其中MOF-d5在77K和1bar时储氢量高达3．7wt％。利用mCT方法研究了氢气在新设计MOF材料中吸附位点，由于向新设计的MOF材料中引入电负性较大的原子，在新设计的MOF材料孔道中以及有机配体周围产生了一些新的吸附位点。
5．采用CBMC和GCMC相结合的方法研究了IRMOF-6和-1对C_4-C_6烷烃异构体混合物的吸附和分离性能。直链烷烃和支链烷烃在IRMOF结构中的吸附量随着压力的增加而增加，高压下支链烷烃的吸附量大于直链烷烃。IRMOF-1和IRMOF-6对C_4-C_6烷烃异构体混合物的相对选择性均不佳，IRMOF-6的选择性略好于IRMOF-1。烷烃分子和IRMOF骨架结构之间的相互作用能的研究表明烷烃分子与Zn_4O团簇之间的相互作用能大于其与有机配体之间的作用能，且烷烃分子越靠近Zn_4O团簇相互作用能越强。由于甲基和有机配体之间的空间位阻所引起的排斥能使得支链烷烃分子很难靠近IRMOF-6的Zn_4O团簇。并分别从热力学和动力学角度解释了烷烃异构体分子在IRMOF-6中的择位吸附现象，研究表明当烷烃异构体混合物主要吸附在孔道之中时，其吸附选择性主要由吸附焓所决定，反之当它们靠近Zn_4O团簇时，吸附选择性则主要由吸附熵所决定。动力学模拟过程中，正丁烷在IRMOF-6的Zn_4O团簇周围的密度和滞留时间略大于异丁烷。
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 (C_4-C_7) 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 L_0 = 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 Zn_4O 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 Zn_4O 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 Zn_4O cluster plays a much more important role than the organic linker during adsorption; and the closer to the Zn_4O 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 Zn_4O 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 (C_4-C_6) 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 C_4-C_6 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 Zn_4O cluster plays much more important role than the organic linker during adsorption; and the closer to the Zn_4O 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 Zn_4O cluster. During molecular dynamics simulation, the number density for n -butane around the Zn_4O cluster of IRMOF-6 is larger than that for 2-methylpropane, and the resident time for one n -butane molecule around the Zn_4O cluster is also slightly longer than that for 2-methylpropane.
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