【作者】 卓胜池；

【导师】 刘洪来； 姜建文；

【作者基本信息】 华东理工大学 ， 物理化学， 2010， 博士

【摘要】 人类活动制造了大量温室气体,使得大气中温室气体的浓度迅速增加,加剧了大气的温室效应,导致全球变暖,并由此引发了越来越多的自然灾害,严重威胁着人类的生存和发展。CO2是主要的温室气体,人类生产和生活中大量燃烧化石燃料,每年向大气中排放数十亿吨的CO2。就目前而言,碳捕集和封存技术(CCS)是最为经济可行的降低CO2排放的方法之一。本文主要针对吸附法捕集CO2技术的关键——多孔吸附材料的开发,采用计算机模拟方法,研究了CO2在微孔、介孔、以及微孔／介孔复合材料中的吸附性质和扩散性质。本文还对表面活性剂在水溶液中的聚集行为以及在气液界面的热缩冷涨现象进行了模拟,并提出用表面活性剂胶束捕集CO2的构想。本文构筑了介孔MCM-41,微孔分子筛MFI和微孔／介孔复合物MFI/MCM-41三种硅基多孔吸附剂,并研究它们在吸附分离CO2方面的性能。本文模拟了CO2在MCM-41中的吸附等温线与吸附热,以及CO2、N2和CH4在MFI中的吸附等温线和吸附热,上述模拟结果都与文献值吻合良好。模拟结果表明,CO2分子与硅基多孔吸附剂的分子间相互作用比CH4和N2强,因此相同条件下的吸附量和吸附热高于CH4和N2。CO2在活性吸附点附近的吸附密度远大于其它吸附位,而N2则倾向于均匀的吸附在吸附剂表面。在压强较低时,CO2在上述三种吸附剂中的吸附量顺序为：MFI>MFI/MCM-41>MCM-41;随着压强升高,CO2在MFI中的吸附趋于饱和,而MCM-41和MFI/MCM-41有较大的介孔,可以容纳更多的吸附质,因此在压强较高时,CO2的吸附量顺序变成：MFI/MCM-41>MCM-41>MFI。CH4在不同孔径的吸附剂中的吸附行为与CO2类似。在本文研究的压强范围内,N2在上述三种吸附剂中的吸附量几乎相等。当气体在MFI/MCM-41中吸附时,吸附质分子首先填充微孔,然后填充介孔。在混合气体吸附时,混合物中各组分的吸附量和吸附热都低于相同条件下纯气体的吸附量和吸附热。其中,与吸附剂相互作用较强的组分(如CO2)只是轻微下降,而与吸附剂相互作用较弱的组分(如CH4,N2)则出现明显的降低。吸附剂对烟道气中的CO2的吸附选择性的顺序依次为：MFI>MFI/MCM-41>MCM-41.在室温下,MFI和MFI/MCM-41对CO2的吸附选择性随压强增大而提高,但是MCM-41对CO2的选择性则呈下降趋势。随着烟道气中CO2含量的增加,吸附剂对CO2的吸附选择性也随之提高。随着温度的升高,气体的吸附量和选择性都迅速下降；高温下,气体的吸附选择性几乎不受压强的影响。温度是气体吸附与分离的主导因素,压强的影响只有在温度较低时才表现出来。在天然气的吸附分离过程中(T=300K),MFI对CO2的选择性随压强的增加先增大后减小；而MFI/MCM-41对CO2的选择性则随压强单调递增,并在一定的压力时超过MFI。CH4与吸附剂的相互作用较弱,因此它在多孔吸附剂中的扩散速率大于CO2。CO2,CH4在MFI中的自扩散系数随吸附量的增加而下降；而它们在MFI/MCM-41自扩散系数则随吸附量的增加先增大后减小,分别在吸附量为2 mmol／g和4 mmol／g附近出现极大值。由于介孔的存在,气体在MFI/MCM-41中的扩散速率远远高于它们在MFI中的扩散速率。对于硅基多孔吸附剂,中低压下微孔对气体的吸附作用大于介孔,其选择性也更高。而在高压下,介孔吸附量大于微孔,传质阻力则小于微孔,有利于吸附质的扩散和传递。模拟表明,在微孔吸附剂中引入介孔可以有效地解决微孔吸附剂传质慢的难题,而且将这种微孔／介孔复合物应用于高压天然气分离时,能够同时提高气体的吸附量,选择性以及扩散速率,其吸附分离性能全面超越微孔分子筛。因此,微孔／介孔复合材料是一种潜在的高效吸附分离材料。对CO2在不同Si／Al比的Na-ZSM-5分子筛中吸附行为的模拟表明：Na+的引入使ZSM-5分子筛出现了强吸附位,CO2在Na+附近的吸附密度和吸附热均高于CO2在ZSM-5表面的吸附。随着Na+含量的增加,ZSM-5对CO2的吸附能力增强。气相压强较低时,分子筛对CO2的吸附几乎全都来自于Na+的贡献；随着压强的升高,由Na+引起的吸附量在总吸附量中所占的比重逐渐下降。本文还提出了用表面活性剂胶束捕集CO2的构想。为了验证这一构想的可行性,本文采用分子动力学模拟研究了CO2在表面活性剂AOT4水溶液中的分布情况,并与韩布兴等人的实验进行了比较。模拟发现,CO2分子首先吸附在AOT4双分子层的酯基周围,随着体系中CO2分子的增加,AOT双分子层的中间逐渐形成CO2的富集区。由于CO2溶胀其中,AOT双分子层变得疏松和不稳定,并且随着增溶CO2数量的增加而逐渐膨胀,在表面张力的驱使下,层状相转变为其它构型。总的来说,模拟结果与韩布兴等人的实验现象比较吻合,说明表面活性剂水溶液中的CO2分子能够在胶束相富集,间接的证明了用表面活性剂捕集CO2具有较高的可行性。但是AOT表面活性剂与CO2的亲和性并不好,可以用具有CO2活性的表面活性剂来捕集CO2。用表面活性剂胶束捕集分离CO2的关键在于寻找或设计出能够形成具有很高CO2吸附量的胶束的表面活性剂。这一构想也许能够为碳捕集和封存(CCS)技术提供一种新的节能高效的CO2捕集方法,但是还有待深入研究探讨。Gemini表面活性剂(17PyOx)在空气／水界面上形成的Langmuir单分子膜具有反常的热缩冷胀现象。本文结合分子动力学模拟和密度泛涵理论的研究表明,17PyOx在水面上的分布是不均匀的,即使在疏松的单分子层中,表面活性剂分子也趋向于聚集在一起。随着表面活性剂的比表面积减小,表面活性剂的尾链沿界面法线方向取向排列,有序度越来越高。疏松的单分子膜对侧壁施加的表面压几乎为零,随着比表面积的减小而呈现微弱的线性增加趋势,而且表面压随温度的升高而增大,表现出正常的热胀冷缩性质。但是对于紧密单分子膜来说,表面压随比表面积的减小迅速增大,而且在278K至293K范围内随温度升高而减小,表现出反常的热缩冷胀现象。17PyOx单分子膜在278K至293K范围内出现的热缩冷胀现象是由于表面活性剂分子间氢键数量随温度升高而增加而导致的。当温度超过293K时,17PyOx的亲水性随温度升高而减弱,以至不能在水面上有效的铺张形成单分子膜。更多还原

【Abstract】 Huge amounts of greenhouse gases have been emitted because of human activities. Greenhouse effect is aggravated by the rapid increase concentration of greenhouse gases in atmosphere, which will lead to the acceleration of global warming followed by severe climate disasters. Therefore, strong measures have to be carried out immediately to reduce the emission of greenhouse gases in order to restrain the influences of global warming on the natural environment. Carbon dioxide is the main greenhouse gas and billions tons of CO2 are emitted due to the combustion of fossil fuel (e.g. coal, petroleum, natural gas) by human activities. Currently, carbon capture and sequestration (CCS) is considered to be the most cost-effective and technically feasible method to reduce the emission of CO2. There are several ways to capture CO2, such as absorption, membrane separation, adsorption, cryogenic distillation. Adopting computer simulation method, we have studied the adsorption and separation of CO2 by porous materials and made a proposal of capturing CO2 by using surfactant micelle.Three adsorbents models, mesoporous MCM-41, microporous MFI and micro/mesoporous MFI/MCM-41, were constructed and studied for CO2 adsorption and separation. The simulated adsorption isotherms and isosteric heats of CO2 in MCM-41, CO2, N2 and CH4 in MFI are consistent well with literature values. Interaction between CO2 and silicate adsorbents is stronger than CH4 and N2, which leads to higher adsorption loading and isosteric heat for CO2 adsorption. The adsorption density of CO2 on active sites is much greater than that on the other area of adsorbent while N2 tends to distributes homogeneously on the adsorbent surface. At low pressure, the loading of CO2 in the three adsorbents is in the following sequence:MFI>MFI/MCM-41>MCM-41. With the increase of pressure, saturated adsorption of MFI gradually approaches while MCM-41 and MFI/MCM-41 are able to accommodate additional adsorbate molecules due to their bigger mesopores. Hence, the sequence of CO2 loading at high pressure changes into MFI/MCM-41>MCM-41>MFI. The adsorption behavior of CH4 in the three adsorbents is similar to CO2. However, throughout the pressure range in our research, the loadings of N2 in the three adsorbents are almost the same. For the adsorption in MFI/MCM-41, adsorbate molecules first locate in micropores and subsequently adsorb in the mesopores.The loading and isosteric heat of each component in mixture are lower than those of pure gas at the same adsorption conditions. In the adsorption of flue gas, the adsorption selectivity of CO2 over N2 is in the sequence of MFI>MFI/MCM-41>MCM-41. At room temperature, the selectivities of MFI and MFI/MCM-41 increase with the rise of pressure while selectivity of MCM-41 decreases. With the increase of temperature, both the gas loading and selectivity decrease rapidly. At high temperature, pressure has no discernible influence on the selectivity. Temperature is the dominant factor for the gas adsorption and selectivity. For the adsorption and separation of natural gas at 300K, with the increase of pressure, the selectivity of CO2 over N2 in MFI increases first and drops later while selectivity in MFI/MCM-41 is a monotonically increasing function of pressure and exceeds the selectivity in MFI at high pressure.Self diffusivity of CH4 is greater than CO2 due to the weaker interaction between CO2 and adsorbent. With the increase of loading, both the self diffusivity of CH4 and CO2 decrease and approach to zero near saturated adsorption. However, the self diffusivities of CH4 and CO2 in MFI/MCM-41 first increase and drop subsequently, experiencing the maximum at the loading about 4mmol/g and 2mmol/g, respectively. Gas diffusion in MFI/MCM-41 is much higher than that in MFI due to the presence of mesopores.For silicate adsorbents, in general, micropore is more adsorptive and selective than mesopore at low and medium pressure. Mesopore is bigger in size than micropore, which leads to higher adsorption capacity at high pressure and low mass transportation resistance for mesopore. By introducing mesopores into microporous adsorbents, it is found to be a good way to solve the problem of low mass transportation rate in microporous materials. This micro/mesoporous composite is able to simultaneously enhance the adsorption capacity, selectivity and mass transportation rate. Micro/mesoporous material is one of the best candidates of high performance adsorbents.The adsorption behavior of CO2 in Na-ZSM-5 zeolites with various Si/Al ratio was also investigated. Strong adsorption sites are introduced with the presence of Na+. Both of the adsorption density and isosteric heat at these sites are greater than the common sites. At low pressure, almost all the adsobate molecules are adsorbed at the strong sites while the contribution of these site decreases with the increase of pressure.We also proposed an idea of capturing the CO2 by surfactant micelle. In order to verify the feasibility of this proposal, molecular dynamics simulation was adopted to investigate the distribution of CO2 in aqueous solution of surfactant AOT. The simulation results were compared with experimental work of Prof. Han Buxing’s group. CO2 molecules first adsorb around the ester group of AOT bilayer. With the increase numbers of CO2 added into the system, CO2 accumulate in the center of bilayer, which leads to the expansion of AOT bilayer. Driven by the surface tension, lamellar bilayer changes to spherical micelle. Both of the simulation result and experimental work demonstrate that CO2 molecules are able to aggregate in micelle phase, which validates our proposal indirectly. However, AOT doesn’t have good affinity to CO2. Several CO2-active surfactants are promoted to capture CO2. The key point of the idea of capturing CO2 with surfactant micelle is to find out or design a suitable surfactant that is able to form micelle with high CO2 capacity. This proposal is expected to be an innovated high efficient method for capturing CO2 but further investigations are still required.Molecular dynamics simulation was also adopted to unravel the negative thermal expansion of Langmuir monolayer formed by gemini surfactant (17PyOx) at air/water interface. Density distribution function along normal direction of water surface validates the formation of monolayer.17PyOx tends to aggregate rather than distribute homogeneously even at loose monolayer. Hydrocarbon tails of 17PyOx become more orderly at the normal direction of monolayer as closer packing. For a loose Langmuir monolayer, surface pressure is around zero and increased linearly and slowly with the decrease of specific area. The monolayer expands positively with rising temperature. As to a compact Langmuir monolayer, however, surface pressure increases rapidly with the decline of specific area and negative thermal expansion occurs. Surface pressure is in decline with the increase of temperature from 278 to 293 K. Negative thermal expansion at this temperature range is due to the increase of hydrogen bond between surfactant-surfactant with the rise of temperature. It is observed by experiment that 17PyOx can not spread well on air/water interface as temperature greater than 298 K. This phenomenon is well predicted by our simulation that hydrophilic head group of 17PyOx becomes hydrophobic at high temperature.更多还原